Problem:
Failing Windows CI:
    FAILED   test/functional\options\defaults_spec.lua @ 361: XDG defaults with too long XDG variables are correctly set
    test\helpers.lua:134: Pattern "Failed to start server: no such file or directory: /X/X/X" not found in log (last 10 lines): Xtest-defaults-log:
    FAILED   test/functional\options\defaults_spec.lua @ 435: XDG defaults with XDG variables that can be expanded are not expanded
    test\helpers.lua:134: Pattern "Failed to start server: no such file or directory: %$XDG_RUNTIME_DIR%/" not found in log (last 10 lines): Xtest-defaults-log:

Solution:
The assert_log() statements are not relevant on Windows, because there
XDG_RUNTIME_DIR is not used for creating servers, it uses \\.pipe\….

Problem:
Tests that _intentionally_ fail certain conditions cause noise in
$NVIM_LOG_FILE:

    $NVIM_LOG_FILE: /home/runner/work/neovim/neovim/build/.nvimlog
    (last 100 lines)
    WRN 2023-01-16T18:26:27.673 T599.7799.0 unsubscribe:519: RPC: ch 1: tried to unsubscribe unknown event 'doesnotexist'
    WRN 2023-01-16T18:29:00.557 ?.11151    server_start:163: Failed to start server: no such file or directory: /X/X/X/...
    WRN 2023-01-16T18:33:07.269 127.0.0.1:12345 server_start:163: Failed to start server: address already in use: 127.0.0.1
    ...
    -- Output to stderr:
    module 'vim.shared' not found:
    	no field package.preload['vim.shared']
    	no file './vim/shared.lua'
    	no file '/home/runner/nvim-deps/usr/share/lua/5.1/vim/shared.lua'
    	no file '/home/runner/nvim-deps/usr/share/lua/5.1/vim/shared/init.lua'
    	no file '/home/runner/nvim-deps/usr/lib/lua/5.1/vim/shared.lua'
    	no file '/home/runner/nvim-deps/usr/lib/lua/5.1/vim/shared/init.lua'
    	no file './vim/shared.so'
    	...
    E970: Failed to initialize builtin lua modules

Solution:
- Log to a private $NVIM_LOG_FILE in tests that intentionally fail and
  cause ERR log messages.
- Assert that the expected messages are actually logged.

Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.

Co-authored-by: Raphael <glephunter@gmail.com>
Co-authored-by: smjonas <jonas.strittmatter@gmx.de>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

- adapt to parser changes from https://github.com/vigoux/tree-sitter-vimdoc/pull/16
- numerous other generator improvements

Add 'mousescroll' option to control how many lines to scroll by when a
mouse wheel keycode is received. The mousescroll option controls both
horizontal and vertical scrolling. The option is a string in the format:

    set mousescroll=direction:count,direction:count

Where direction is either "ver" or "hor", and count is a non negative
integer. If a direction is omitted, a default value is used. The default
values remain unchanged, that is 3 for vertical scrolling, and 6 for
horizontal scrolling. As such, the mousescroll default is "ver:3,hor:6".

Add mousescroll documentation
 - Add option documentation in options.txt
 - Add brief summary in quickref.txt

Update :help scroll-mouse-wheel
 - Mention mousescroll option as a means of controlling scrolling.
 - Remove obsolete suggestion to map scroll wheel keys to <C-U> to
   scroll by a single line -- users should prefer the mousescroll option.
 - Add some information about the consequences of remapping scroll wheel
   keys (they lose their magic ability to affect inactive windows).

Update :help vim-differences
 - Add brief mousescroll summary under Options

Add mousescroll tests
 - Test option validation
 - Test default mousescroll value and behavior
 - Test fallback to default values
 - Test mouse vertical and horizontal scrolling in normal mode
 - Test mouse vertical and horizontal scrolling in insert mode

Problem:
- Since c57f6b28d71d #8519, sockets are created in ~/.local/… but XDG
  spec says: "XDG_RUNTIME_DIR: Must be on the local filesystem", which
  implies that XDG_STATE_DIR is potentially non-local.
- Not easy to inspect Nvim-created temp files (for debugging etc).

Solution:
- Store sockets in stdpath('run') ($XDG_RUNTIME_DIR).
- Establish "/tmp/nvim.user/" as the tempdir root shared by all Nvims.
- Make ok() actually useful.
- Introduce assert_nolog().

closes #3517
closes #17093