Improve timeline entries for WIP prefix changes in pull requests (#36518)

Add new timeline event types when the WIP prefix is added or removed,
replacing the previous ugly title change messages.

Fixes: https://github.com/go-gitea/gitea/issues/36517

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

options/locale/locale_en-US.json

@@ -1778,6 +1778,8 @@
   "repo.pulls.title_desc": "wants to merge %[1]d commits from <code>%[2]s</code> into <code id=\"branch_target\">%[3]s</code>",
   "repo.pulls.merged_title_desc": "merged %[1]d commits from <code>%[2]s</code> into <code>%[3]s</code> %[4]s",
   "repo.pulls.change_target_branch_at": "changed target branch from <b>%s</b> to <b>%s</b> %s",
+  "repo.pulls.marked_as_work_in_progress_at": "marked the pull request as work in progress %s",
+  "repo.pulls.marked_as_ready_for_review_at": "marked the pull request as ready for review %s",
   "repo.pulls.tab_conversation": "Conversation",
   "repo.pulls.tab_commits": "Commits",
   "repo.pulls.tab_files": "Files Changed",