feat(treesitter)!: don't parse tree in get_parser() or start()

**Problem:** `vim.treesitter.get_parser()` and `vim.treesitter.start()`
both parse the tree before returning it. This is problematic because if
this is a sync parse, it will stall the editor on large files. If it is
an async parse, the functions return stale trees.

**Solution:** Remove this parsing side effect and leave it to the user
to parse the returned trees, either synchronously or asynchronously.

runtime/lua/vim/treesitter/highlighter.lua

@@ -148,8 +148,6 @@ function TSHighlighter.new(tree, opts)
     vim.opt_local.spelloptions:append('noplainbuffer')
   end)
 
-  self.tree:parse(nil, function() end)
-
   return self
 end