perf(highlight): Skip redraw over folded lines

Fixes: folke#285

Currently on a window update such as `WinScroll` highlight_win is
looping through every line from from `w0` to `w$` to update the
highlights. On large files with very large folded regions these two
lines can be very far apart and highlighting every single line can be
costly even though they are not in view. This update adds a check to
skip over any lines hidden in a fold.

lua/todo-comments/highlight.lua

@@ -101,8 +101,14 @@ function M.redraw(buf, first, last)
   first = math.max(first - Config.options.highlight.multiline_context, 0)
   last = math.min(last + Config.options.highlight.multiline_context, vim.api.nvim_buf_line_count(buf))
   local state = M.get_state(buf)
-  for i = first, last do
-    state.dirty[i] = true
+  while first <= last do
+    -- skip over lines that are hidden under a fold
+    if vim.fn.foldclosed(first) == -1 then
+      state.dirty[first] = true
+    else
+      first = vim.fn.foldclosedend(first)
+    end
+    first = first + 1
   end
   if not M.timer then
     M.timer = vim.defer_fn(M.update, Config.options.highlight.throttle)