Add support for .markdown extension (#13609)

Fixes #13608

Release Notes:

- Added recognizing *.markdown files as Markdown
([#13608](https://github.com/zed-industries/zed/issues/13608)).
891f195f7bPeter Tripp committed on 6/28/2024, 12:34:06 PM· committed by GitHubparent6e1b99b
2 files changedLine totals unavailable