This pull request add support for Ruby singleton methods in the document
outline.

**Before**

<img width="1490" alt="outline-before"
src="https://github.com/zed-industries/zed/assets/503025/e90afd4c-bcdd-477b-92cf-fe34cec54980">

**After**

<img width="1493" alt="outline-after"
src="https://github.com/zed-industries/zed/assets/503025/067cc80b-7718-4395-9079-4677a689b9be">

Release Notes:

- Added support for singleton methods in Ruby outline view. Fixed
[#1442](https://github.com/zed-industries/community/issues/1442).