[optional] argument

[justinmk]

Note: do a release before merging this

---

• Parse [foo] as optional ~~optional_arg~~ (naming suggestions?)
• Don't treat [{arg}] as optional, else it clobbers the nested {arg}.

fix #1

Todo

• ✅ Will need to work through similar issues as for {argument}. These examples cause errors:

  nvim_buf_changedtick_event[{buf},]
  A list of [text, hl_group] arrays
  to change) ["win", winid, bufnr, row]
  

  • ✅ Fixed by defined optional as a regex instead of using _word().