fix(treesitter): get_captures_at_position returns metadata
Return the full `metadata` table for the capture instead of just the
priority.
Further cleanup of related docs.
The @error capture is used for tree-sitter's ERROR node, which indicates
a parsing error -- which can be quite frequent (and jarring) while typing.
Users can still manually `hi link @error Error` in their config.
This removes the support for defining links via
vim.treesitter.highlighter.hl_map (never documented, but plugins did
anyway), or the uppercase-only `@FooGroup.Bar` to `FooGroup` rule.
The fallback is now strictly `@foo.bar.lang` to `@foo.bar` to `@foo`,
and casing is irrelevant (as it already was outside of treesitter)
For compatibility, define default links to builting syntax groups
as defined by pre-existing color schemes
