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:
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.

A lot of functions in move.c only worked for curwin, alternatively
took a `wp` arg but still only work if that happens to be curwin.

Refactor those that are needed for update_topline(wp) to work
for any window.

fixes #27723
fixes #27720

Problem: Many places in the code use `findoption()` to access an option using its name, even if the option index is available. This is very slow because it requires looping through the options array over and over.

Solution: Use option index instead of name wherever possible. Also introduce an `OptIndex` enum which contains the index for every option as enum constants, this eliminates the need to pass static option names as strings.