Enable EmmyLua's need-check-nil diagnostic and resolve nullable values
through more precise types, explicit invariants, and guards for missing
runtime state. Keep analyzer-specific workarounds local and update the
affected API documentation.

Exclude the bundled re.lua from analysis so its nil-check warnings do
not fail CI.

AI-assisted

Enable checks for unused code, deprecated calls, return types, and
annotations.

Fix the warnings and replace the deprecated highlight function. Keep
exceptions where old APIs are still needed or the checker gets it wrong.

AI-assisted

This automatically downloads and uses the correct luals binary for the
currently used system. `make luals` will run luals on all lua files in
`runtime`.

We download lua-language-server manually instead of relying on
contributors downloading it on their own (like with stylua) as
lua-language-server is updated frequently which may cause unnecessary
friction. Therefore, we download a pinned version of luals which we then
can manually bump when needed. This can be re-evaluated if luals becomes
more stable in the future.

Currently this is not run when using `make lint` since cmake style "file
caching" doesn't seem possible at the moment. This is because checking a
single file doesn't seem to work.

Work on https://github.com/neovim/neovim/issues/24563.

Problem:
Can't use `n.clear()` to test non-RPC `nvim` invocations. So tests end
up creating ad-hoc wrappers around `system()` or `jobstart()`.

Solution:
- Introduce `n.spawn_wait()`
- TODO (followup PR): Rename `n.spawn()` and `n.spawn_wait()`.
  It's misleading that `n.spawn()` returns a RPC session...

Do not require luarocks on PUC lua CI just because of this single lua file