Problem: Option validators and setters repeatedly pass error buffers and
their sizes separately. Some setters return messages in static storage.

Solution:

- Add CharBuf for borrowed writable storage, with its length including
  space for a terminating NUL.
- Move option error buffers to the callers that report errors. Preserve
  existing buffer sizes, truncation, and calls that do not need a
  message.
- Consolidate option error formatting and the duplicated code that lists
  valid option values.
- Return error messages directly from do_one_set_option().
- Remove unused arguments from get_option_newval() and
  set_option_value_for().
- Handle legacy TTY assignments in the existing callers and remove the
  redundant compatibility wrapper.
- Reuse CharBuf for Tree-sitter query errors.
- Pass CharBuf through the statusline and statuscolumn builders,
  retaining the buffer sizes provided by their callers, including title
  formatting.

AI-assisted

Problem:
Assigning to the local value of a global-local boolean option
('autoread', 'autocomplete', 'fsync') aborts:

    Assertion failed: (curval.type == newval.type), function
    ex_let_option, file vars.c, line 1408.

    ex_let_one
    ex_let_vars
    ex_let
    execute_cmd0
    do_cmdline
    call_user_func
    ...
    eval_map_expr
    vgetorpeek
    vgetc
    state_enter
    main

A global-local option may have local value `kObjectTypeUnset`, but
`ex_let_option()` guards only `kObjectTypeNil`.

Solution:
When curval is Unset, resolve it to the inherited global value.

Problem:
Assigning a Boolean or special value (v:true/v:false/v:null/v:none) to a TTY
option aborts:

    Assertion failed: (curval.type == newval.type), function ex_let_option, file vars.c, line 1408.
    3   libsystem_c.dylib   __assert_rtn + 284
    4   nvim                ex_let_one + 3308
    5   nvim                ex_let_vars + 112
    6   nvim                ex_let + 2356
    7   nvim                execute_cmd0 + 252
    8   nvim                do_cmdline + 9076
    9   nvim                call_user_func + 3320
    10  nvim                call_func + 2076
    11  nvim                get_func_tv + 696
    12  nvim                eval_func + 380
    20  nvim                eval_to_string_eap + 276
    21  nvim                eval_map_expr + 444
    22  nvim                vgetorpeek + 3172
    23  nvim                vgetc + 764

Solution:
Apply the string-type check to TTY options too, so a Boolean/special value
gives "E928: String required" instead of aborting. Valid string/number
assignments to `t_*` pseudo-options still silently no-op.

Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.

- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
  merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
  "know" about the various magic symbols.

So they harm DX, which means they serve no purpose at all.

Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
  `t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.

Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.

Closes https://github.com/neovim/neovim/issues/27004.

Work on https://github.com/neovim/neovim/issues/27004.