fix(treesitter): make foldexpr work without highlighting (#24167)

Problem: Treesitter fold is not updated if treesitter hightlight is not
active. More precisely, updating folds requires `LanguageTree:parse()`.

Solution: Call `parse()` before computing folds and compute folds when
lines are added/removed.

This doesn't guarantee correctness of the folds, because some changes
that don't add/remove line won't update the folds even if they should
(e.g. adding pair of braces). But it is good enough for most cases,
while not introducing big overhead.

Also, if highlighting is active, it is likely that
`TSHighlighter._on_buf` already ran `parse()` (or vice versa).

runtime/lua/vim/treesitter/_fold.lua

@@ -162,9 +162,7 @@ local function get_folds_levels(bufnr, info, srow, erow)
 
   local parser = ts.get_parser(bufnr)
 
-  if not parser:is_valid() then
-    return
-  end
+  parser:parse()
 
   parser:for_each_tree(function(tree, ltree)
     local query = ts.query.get(ltree:lang(), 'folds')
@@ -283,10 +281,12 @@ local function on_bytes(bufnr, foldinfo, start_row, old_row, new_row)
   local end_row_old = start_row + old_row
   local end_row_new = start_row + new_row
 
+  if new_row ~= old_row then
     if new_row < old_row then
       foldinfo:remove_range(end_row_new, end_row_old)
-  elseif new_row > old_row then
+    else
       foldinfo:add_range(start_row, end_row_new)
+    end
     schedule_if_loaded(bufnr, function()
       get_folds_levels(bufnr, foldinfo, start_row, end_row_new)
       recompute_folds()

test/functional/treesitter/parser_spec.lua

@@ -922,12 +922,6 @@ int x = INT_MAX;
       [19] = '1' }, get_fold_levels())
 
     helpers.command('1,2d')
-    helpers.poke_eventloop()
-
-    exec_lua([[vim.treesitter.get_parser():parse()]])
-
-    helpers.poke_eventloop()
-    helpers.sleep(100)
 
     eq({
       [1] = '0',
@@ -947,6 +941,29 @@ int x = INT_MAX;
       [15] = '2',
       [16] = '1',
       [17] = '0' }, get_fold_levels())
+
+    helpers.command('1put!')
+
+    eq({
+      [1] = '>1',
+      [2] = '1',
+      [3] = '1',
+      [4] = '1',
+      [5] = '>2',
+      [6] = '2',
+      [7] = '2',
+      [8] = '1',
+      [9] = '1',
+      [10] = '>2',
+      [11] = '2',
+      [12] = '2',
+      [13] = '2',
+      [14] = '2',
+      [15] = '>3',
+      [16] = '3',
+      [17] = '3',
+      [18] = '2',
+      [19] = '1' }, get_fold_levels())
   end)
 
   it('tracks the root range properly (#22911)', function()