fix(treesitter): ensure syntaxset augroup exists (#29542)

Problem:
Error when calling vim.treesitter.start() and vim.treesitter.stop() in
init.lua.

Solution:
Ensure syntaxset augroup exists after loading synload.vim.

runtime/lua/vim/treesitter/highlighter.lua

@@ -139,8 +139,11 @@ function TSHighlighter.new(tree, opts)
   -- but use synload.vim rather than syntax.vim to not enable
   -- syntax FileType autocmds. Later on we should integrate with the
   -- `:syntax` and `set syntax=...` machinery properly.
+  -- Still need to ensure that syntaxset augroup exists, so that calling :destroy()
+  -- immediately afterwards will not error.
   if vim.g.syntax_on ~= 1 then
     vim.cmd.runtime({ 'syntax/synload.vim', bang = true })
+    vim.api.nvim_create_augroup('syntaxset', { clear = false })
   end
 
   vim._with({ buf = self.bufnr }, function()