fix(treesitter): recalculate folds on VimEnter #32240

**Problem:** In the case where the user sets the treesitter foldexpr upon startup in
their `init.lua`, the fold info will be calculated before the parser has
been loaded in, meaning folds will be properly calculated until edits or
`:e`.

**Solution:** Refresh fold information upon `VimEnter` as a sanity check
to ensure that a parser really doesn't exist before always returning
`'0'` in the foldexpr.

runtime/lua/vim/treesitter/_fold.lua

@@ -380,7 +380,7 @@ function M.foldexpr(lnum)
 
   if not foldinfos[bufnr] then
     foldinfos[bufnr] = FoldInfo.new(bufnr)
-    api.nvim_create_autocmd('BufUnload', {
+    api.nvim_create_autocmd({ 'BufUnload', 'VimEnter' }, {
       buffer = bufnr,
       once = true,
       callback = function()