fix(treesitter): do not map hl_group when no mapping is set

This changes the behavior of the hl_cache to the old one.

- when the capture exists as a hlgroup -> use it
- when hl_map contains a mapping -> use it
- else do nothing (before: map capture to non-existing capture)

Before also captures `@foo.bar` would intend to use the hlgroup `foo.bar`
which results in a confusing error since hlgroups can't contain dots.

runtime/lua/vim/treesitter/highlighter.lua

@@ -116,7 +116,7 @@ function TSHighlighterQuery:_get_hl_from_capture(capture)
     -- From "Normal.left" only keep "Normal"
     return vim.split(name, '.', true)[1], true
   else
-    return TSHighlighter.hl_map[name] or name, false
+    return TSHighlighter.hl_map[name] or 0, false
   end
 end