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/lua/treesitter.c

@@ -489,7 +489,11 @@ static int parser_parse(lua_State *L)
   // Sometimes parsing fails (timeout, or wrong parser ABI)
   // In those case, just return an error.
   if (!new_tree) {
-    return luaL_error(L, "An error occurred when parsing.");
+    if (ts_parser_timeout_micros(p) == 0) {
+      // No timeout set, must have had an error
+      return luaL_error(L, "An error occurred when parsing.");
+    }
+    return 0;
   }
 
   // The new tree will be pushed to the stack, without copy, ownership is now to the lua GC.

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