When treesitter is enabled, by default syntax groups are not defined, but these
groups are used to identify where to skip matches in matchit and matchparen.
This patch does three things:
1. If syntax is enabled regardless of treesitter (`vim.bo.syntax='on'`):
Use original implementation.
2. If treesitter is enabled and syntax is not:
Match the syntax groups (i.e. `comment\|string`) against treesitter captures
to check for skipped groups.
3. Add an explicit treesitter syntax for marking captures to skip:
matchit uses `b:match_skip` to determine what counts as skippable
Where 's:comment\|string' uses a match of the named syntax groups against
a regex match of comment\|string, 't:comment\|string' now uses vim regex
to match against the names of the treesitter capture groups.
