feat(treesitter): async parsing

**Problem:** Parsing can be slow for large files, and it is a blocking
operation which can be disruptive and annoying.

**Solution:** Provide a function for asynchronous parsing, which accepts
a callback to be run after parsing completes.

Co-authored-by: Lewis Russell <lewis6991@gmail.com>
Co-authored-by: Luuk van Baal <luukvbaal@gmail.com>
Co-authored-by: VanaIgr <vanaigranov@gmail.com>

src/nvim/options.lua

@@ -6520,8 +6520,8 @@ return {
       defaults = { if_true = 2000 },
       desc = [=[
         Time in milliseconds for redrawing the display.  Applies to
-        'hlsearch', 'inccommand', |:match| highlighting and syntax
-        highlighting.
+        'hlsearch', 'inccommand', |:match| highlighting, syntax highlighting,
+        and async |LanguageTree:parse()|.
         When redrawing takes more than this many milliseconds no further
         matches will be highlighted.
         For syntax highlighting the time applies per window.  When over the