Problem:    It is not possible to specify cell widths of characters.
Solution:   Add setcellwidths().
https://github.com/vim/vim/commit/08aac3c6192f0103cb87e280270a32b50e653be1

Co-Authored-By: delphinus <me@delphinus.dev>

In case nvim A sends nvim_error_event to nvim B, it would
respond with another nvim_error_event due to unknown
request name. Fix this by adding dummy request handler for now.

Problem:    Get ml_get error when deleting a line in 'completefunc'. (Yegappan
            Lakshmanan)
Solution:   Lock the text while evaluating 'completefunc'.
https://github.com/vim/vim/commit/ff06f283e3e4b3ec43012dd3b83f8454c98f6639

Fix a mistake in the porting of patch 8.1.0098.
Cherry-pick Test_run_excmd_with_text_locked() from patch 8.2.0270.
Cherry-pick test_gf.vim changes from patch 8.2.0369.
Cherry-pick message change from later patches.

Note for external UIs: Nvim can now emit multiple "redraw" event batches
before a final "flush" event is received. To retain existing behavior,
clients should make sure to update visible state at an explicit "flush"
event, not just the end of a "redraw" batch of event.

* Get rid of copy_object() blizzard in the auto-generated ui_event layer
* Special case "grid_line" by encoding screen state directly to
  msgpack events with no intermediate API events.
* Get rid of the arcane notion of referring to the screen as the "shell"
* Array and Dictionary are kvec_t:s, so define them as such.
* Allow kvec_t:s, such as Arrays and Dictionaries, to be allocated with
  a predetermined size within an arena.
* Eliminate redundant capacity checking when filling such kvec_t:s
  with values.

feat: user command "preview" (like inccommand)

Adds a Lua-only `preview` flag to user commands which allows the command to be incrementally previewed like `:substitute` when 'inccommand' is set.

Problem:
1. The main log routine does not protect itself against recursion.
   log_lock() doesn't guard against recursion, it would deadlock...
2. 22b52dd462e5 (#11501) regressed 6f27f5ef91b3 (#10172), because
   set_init_1..process_spawn tries to log (see backtrace below), but the
   mutex isn't initialized yet. Even if the mutex were valid, we don't
   want early logging to fallback to stderr because that can break
   embedders when stdio is used for RPC.

    frame 1: 0x00000001001d54f4 nvim`open_log_file at log.c:205:7
    frame 2: 0x00000001001d5390 nvim`logmsg(log_level=1, context="UI: ", func_name=0x0000000000000000, line_num=-1, eol=true, fmt="win_viewport") at log.c:150:20
    frame : 0x000000010039aea2 nvim`ui_call_win_viewport(grid=2, win=1000, topline=0, botline=1, curline=0, curcol=0, line_count=1) at ui_events_call.generated.h:321:3
    frame 4: 0x00000001003dfefc nvim`ui_ext_win_viewport(wp=0x0000000101816400) at window.c:939:5
    frame 5: 0x00000001003ec5b4 nvim`win_ui_flush at window.c:7303:7
    frame 6: 0x00000001003a04c0 nvim`ui_flush at ui.c:508:3
    frame 7: 0x00000001002966ba nvim`do_os_system(argv=0x0000600000c0c000, input=0x0000000000000000, len=0, output=0x0000000000000000, nread=0x00007ff7bfefe830, silent=false, forward_output=false) at shell.c:894:3
    frame 8: 0x0000000100295f68 nvim`os_call_shell(cmd="unset nonomatch; vimglob() { while [ $# -ge 1 ]; do echo \"$1\"; shift; done }; vimglob >/var/folders/gk/3tttv_md06987tlwpyp62jrw0000gn/T/nvimwwvwfD/0 ~foo", opts=kShellOptExpand | kShellOptSilent | kShellOptHideMess, extra_args=0x0000000000000000) at shell.c:663:18
    frame 9: 0x0000000100295845 nvim`call_shell(cmd="unset nonomatch; vimglob() { while [ $# -ge 1 ]; do echo \"$1\"; shift; done }; vimglob >/var/folders/gk/3tttv_md06987tlwpyp62jrw0000gn/T/nvimwwvwfD/0 ~foo", opts=kShellOptExpand | kShellOptSilent | kShellOptHideMess, extra_shell_arg=0x0000000000000000) at shell.c:712:14
    frame 10: 0x0000000100294c6f nvim`os_expand_wildcards(num_pat=1, pat=0x00007ff7bfefeb20, num_file=0x00007ff7bfefee58, file=0x00007ff7bfefee60, flags=43) at shell.c:328:7
    ...
    frame 23: 0x000000010028ccef nvim`expand_env_esc(srcp=",~foo", dst="~foo", dstlen=4094, esc=false, one=false, prefix=0x0000000000000000) at env.c:673:17
    frame 24: 0x000000010026fdd5 nvim`option_expand(opt_idx=29, val=",~foo") at option.c:1950:3
    frame 25: 0x000000010026f129 nvim`set_init_1(clean_arg=false) at option.c:558:19
    frame 26: 0x00000001001ea25e nvim`early_init(paramp=0x00007ff7bfeff5f0) at main.c:198:3
    frame 27: 0x00000001001ea6bf nvim`main(argc=1, argv=0x00007ff7bfeff848) at main.c:255:3

Solution:
1. Check for recursion, show "internal error" message.
    - FUTURE: when "remote TUI" is merged, can we remove log_lock()?
2. Skip logging if log_init wasn't called yet.