* docs(options): take ownership of options.txt

- `src/nvim/options.lua` is now the source of truth
- generate runtime/lua/vim/_meta/options.lua

* fixup! zeer comments

* fixup! zeer comments (2)

* fixup! re-enable luacheck

* fixup! regen

- eval.lua is now the source of truth.

- Formatting is much more consistent.

- Fixed Lua type generation for polymorphic functions (get(), etc).

- Removed "Overview" section from builtin.txt
  - Can generate this if we really want it.

- Moved functions from sign.txt and testing.txt into builtin.txt.

- Removed the *timer* *timers* tags since libuv timers via vim.uv should be preferred.

- Removed the temp-file-name tag from tempname()

- Moved lueval() from lua.txt to builtin.txt.

* Fix indent

* fixup!

* fixup! fixup!

* fixup! better tag formatting

* fixup: revert changes no longer needed

* fixup! CI

---------

Co-authored-by: zeertzjq <zeertzjq@outlook.com>

Problem:
  No LSP information for `vim.fn.*`

Solution:
  Add meta file for `vim.fn.*`.

The new oldtest directory is in test/old/testdir. The reason for this is
that many tests have hardcoded the parent directory name to be
'testdir'.

These aren't needed as we don't use codecov anymore.

src/nvim/testdir/runnvim.sh re-uses the test suite code from ci/ to
cleanup the output of legacy tests.

Closes #15856

[skip ci]

These are only relevant for GitHub, not published tarballs.