Besides the "visible" improvements, this release features numerous
internal improvements to the UI/screen code and test infrastructure.

Numerous patches were merged from Vim, which are not mentioned below.

FEATURES:

07ad5d71ab97 clipboard: Support custom VimL functions #9304
725da1feeb9a #9401 win/TUI: Improve terminal/console support
7a8dadbedb0b #9077 startup: Use $XDG_CONFIG_DIRS/nvim/sysinit.vim if it exists
feec926633e3 #9299 support <cmd> mapping in more places
0653ed63a508 #9028 diff/highlight: Show underline for low-priority CursorLine
bddcbbb5716a signs: Add "numhl" argument #9113
05f9c7c2f7f5 clipboard: support Wayland (#9230)
14ae394532aa #9052 TUI: add support for undercurl and underline color
4fa3492a6fcb #9023 man.vim: soft (dynamic) wrap #9023

API:

8b39e4ec7933 #6920 API: implement object namespaces
b1aaa0a881ef API: Implement nvim_win_set_buf() #9100
8de87c7b1ca0 #8180 API: virtual text annotations (nvim_buf_set_virtual_text)
2b9fc9a13f14 #8660 API: add nvim_buf_is_loaded()
    API: buf_get_lines, buf_line_count handle unloaded buffers
88f77c28e52a API: nvim_buf_get_offset_for_line
94841e5eaebc API/UI: #8221 ext_newgrid, ext_hlstate
    (use line-based rather than char-based updates)

UI

b5cfac089409 #8806 TUI: use BCE again more often, (smoother resizes/scrolling)
77b5e9ae25fc #9315 screen: add missing status redraw when redraw_later(CLEAR) was used
5f15788dc3ac TUI: clip invalid regions on resize (#8779), fixes #8774
c936ae0f3688 #9193 TUI: improvements for scrolling and clearing
f20427451e7f #9143 UI: disable clearing almost everywhere
f4b2b6666189 #9079 TUI: always use safe cursor movement after resize
d36afafc8dc7 #9211 ui_options: also send when starting or from OptionSet
67f80d485cec TUI: Avoid reset_cursor_color in old VTE #9191
e55ebae37340 #9021 don't erase screen on `:hi Normal` during startup
c5790d91897c #8915 TUI: Hint wrapped lines to terminals.

FIXES:

231de7253982 RPC: turn errors from async calls into notifications
907ad921bc2f TUI: Restore terminal title via "title stacking" (#9407)
cb76a8a95f3a genappimage: Unset $ARGV0 at invocation #9376
b48efd9ba7bf #9347 TUI: FreeBSD: Improve support for BSD vt console
c16529afa5c8 TUI: Konsole 18.07.70 supports DECSCUSR (#9364)
aec096fc5b8a os/lang: use the correct LC_NUMERIC also for OS X
5fee0be91552 provider: improve error message (#9344)
3c42d7a10a04 TUI: alacritty supports set_cursor_color #9353
7bff9a5de842 TUI: Alacritty supports DECSCUSR (#9048)
57acfceabeb3 macOS: infer primary language if $LANG is empty #9345
bc132ae123d4 runtime/syntax: Fix highlighting of augroup contents (#9328)
715fdfee1ed5 #9297 VimL/confirm(): Show dialog even if :silent
799d9c32157c clipboard: Prefer xclip (#9302)
6dae7776ed0d provider/nodejs: fix npm,yarn detection
16bc1e9c1782 #9218 channel: avoid buffering output when only terminal and no callbacks are active
72fecad1ffe3 #8804 Fix crash in lang_init() on macOS if lang_region = NULL
d58139877990 ruby: detect rbenv shims for other versions (#8733)
e568ac7a68d3 #9123 third-party/unibilium: Fix parsing of extended capability entries
c4c74c3883aa jobstart(): Fix hang on non-executable cwd #9204
1cf50cbfd9b7 provider/nodejs: Simultaneously query npm and yarn #9054
6c496db4b75e undo: Fix infinite loop if undo_read_byte returns EOF #2880
f8f83579ff42 #9034 'swapfile: always show dialog'

CHANGES:

c236e80cf3df #9024 --embed: wait for UI unless --headless
180b50dddc26 #9248 python: 'neovim' module was renamed to 'pynvim'
2000b6a64a9c #8589 VimL: Remove legacy aliases "v:errmsg", "v:shell_error", "v:this_session"
deb18a050ef5 defaults: background=dark #2894 (#9205)
c1187d4af013 defaults: win: 'shellpipe' for cmd.exe (#8827)

vim-patch:8.0.0428: git and hg see new files after running tests
vim-patch:8.0.0429: options test does not always test everything
vim-patch:8.0.0430: options test fails or hangs on MS-Windows
vim-patch:8.0.0436: running the options test sometimes resizes the terminal
vim-patch:8.0.0488: running tests leaves an "xxx" file behind
vim-patch:8.0.0585: test_options fails when run in the GUI

vim-patch:8.1.0558: some MS-Windows instructions are outdated
vim-patch:8.1.0565: asan complains about reading before allocated block
vim-patch:8.1.0566: SGR not enabled for mintty because $TERM is "xterm"
vim-patch:8.1.0567: error for NUL byte in ScreenLines goes unnoticed
vim-patch:8.1.0568: error message for NUL byte in ScreenLines breaks Travis CI
vim-patch:8.1.0577: tabpage right-click menu never shows "Close tab"
vim-patch:8.1.0578: cannot disable arabic, rightleft and farsi in configure
vim-patch:8.1.0587: GvimExt: realloc() failing is not handled properly
vim-patch:8.1.0589: compilation error in gvimext.cpp
vim-patch:8.1.0592: the libvterm tests are not run as part of Vim tests
vim-patch:8.1.0593: illegal memory access in libvterm test
vim-patch:8.1.0594: libvterm tests fail to run on Mac
vim-patch:8.1.0595: libvterm tests are not run with coverage
vim-patch:8.1.0597: cannot run test_libvterm from the top directory
vim-patch:8.1.0598: indent tests may use the wrong Vim binary
vim-patch:8.1.0605: running make in the top directory echoes a comment 
vim-patch:8.1.0606: 'cryptmethod' defaults to a very old method
vim-patch:8.1.0607: proto files are not in sync with the source code
vim-patch:8.1.0608: coverals is not updating
vim-patch:8.1.0609: MS-Windows: unused variable, depending on the Ruby version
vim-patch:8.1.0610: MS-Windows ctags file list differs from Unix
vim-patch:8.1.0616: NSIS installer is outdated
vim-patch:8.1.0617: NSIS installer gets two files from the wrong directory
vim-patch:8.1.0620: overuling CONF_ARGS from the environment no longer works
vim-patch:8.1.0624: overuling CONF_ARGS from the environment still does not work
vim-patch:8.1.0628: Compiler warning on MS-Windows.
vim-patch:8.1.0635: Coverity complains about null pointer use
vim-patch:8.1.0637: nsis file no longer used
vim-patch:8.1.0646: cannot build with Ruby 2.6.0

closes #9320
closes #9128

This was added in 549311db7d400 for some reason, it usually just looks
weird.

The purpose of the {Nvim} hint was not well-defined, and its usage
inconsistent.  It's also unnecessary.

Nvim-Vim differences are centralized at:
    :help vim-differences
Removed things are centralized at:
    :help deprecated
Developer guidelines for documentation are listed at:
    :help dev-doc

- CMake features