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

This introduces the following breaking changes:
- nvim_get_keymap now always returns a LuaRef object as "callback" for a
  Lua mapping regardless of how it is called. The LuaRef object can be
  called from Lua and Vim script, but is lost over RPC.
- maparg() now returns a Funcref instead of a ref number as "callback"
  for a Lua mapping. The Funcref can be called from Lua and Vim script,
  but is lost over RPC.

This may also make nvim_get_keymap faster, but make maparg() slower.

Co-authored-by: ii14 <ii14@users.noreply.github.com>

Problem:
1. Log messages (especially in CI) are hard to correlate with tests.
2. Since b353a5c05f02 #11886, dumplog() prints the logs next to test
   failures. This is noisy and gets in the way of the test results.

Solution:
1. Associate an incrementing id with each test and include it in log
   messages.
    - FUTURE: add v:name so Nvim instances can be formally "named"?
2. Mention "child" in log messages if the current Nvim is a child (based
   on the presence of $NVIM).

BEFORE:

    DBG … 12345      UI: event
    DBG … 12345      log_server_msg:722: RPC ->ch 1: …
    DBG … 12345      UI: flush
    DBG … 12345      inbuf_poll:444: blocking... events_enabled=1 events_pending=0
    DBG … 23454      UI: stop
    INF … 23454      os_exit:594: Nvim exit: 0

AFTER:

    DBG … T57        UI: event
    DBG … T57        log_server_msg:722: RPC ->ch 1: …
    DBG … T57        UI: flush
    DBG … T57        inbuf_poll:444: blocking... events_enabled=1 events_pending=0
    DBG … T57/child  UI: stop
    INF … T57/child  os_exit:594: Nvim exit: 0

Also avoid referring to mappings as "keymaps" in commands and docs.

						*map_empty_rhs* *map-empty-rhs*
You can create an empty {rhs} by typing nothing after a single CTRL-V (you
have to type CTRL-V two times).  Unfortunately, you cannot do this in a vimrc
file.

Problem:    Cannot map <C-H> when modifyOtherKeys is enabled.
Solution:   Add the <C-H> mapping twice, both with modifier and as 0x08.  Use
            only the first one when modifyOtherKeys has been detected.
https://github.com/vim/vim/commit/459fd785e4a8d044147a3f83a5fca8748528aa84

Add REPTERM_NO_SPECIAL instead of REPTERM_SPECIAL because the meaning of
"special" is different between Vim and Nvim.
Omit seenModifyOtherKeys as Nvim supports attaching multiple UIs.
Omit tests as they send terminal codes.
Keep the behavior of API functions.