Problem: get_captures_at_pos() only returns captures at one position. It
cannot query a whole line or range. It can also miss captures from
injected languages outside the visible area.

Solution: Add vim.treesitter.get_captures() with a required start and an
optional exclusive stop. Accept row numbers or {row, col} positions.
Without a stop, query one position or a whole line. Parse the requested
range and return capture coordinates.

Deprecate get_captures_at_pos() with a compatibility wrapper and migrate
its internal callers. Keep get_captures_at_cursor() for capture names at
a window's cursor.

AI-assisted

Problem:
Cannot assign Lua functions/closures to "func" ('completefunc',
'tagfun', …) or "expr" ('foldexpr', 'indentexpr', …) options.

Solution:
- Store "func"/"expr" options as `Callback` instead of string.
- Delete oceans of copy-pasted code.
- BREAKING: LuaRef returned via RPC/Vimscript is now represented as
  `"<Lua N: file:line>"` (like what `:map` shows) instead of `nil`.
- Note: `man.vim` still uses `v:lua` string, bc it's a vimscript ftplugin.

Helped-by: Lewis Russell <lewis6991@gmail.com>

Type fixes caught by emmylua

* refactor(docs): remove `@private` annotations from local functions

* refactor(docs): remove unnecessary `@nodoc` annotations

**Problem:** Some weird languages have different comment syntax
depending on the location in the code, and we do not have a way to
determine the correct `commentstring` for these special cases.

**Solution:** Allow queries to specify `commentstring` values in
metadata, allowing users/`nvim-treesitter` to provide a better
commenting experience without hugely increasing the scope of the code in
core.