ruby: Add a new injection for regular expressions (#12533)

# Summary

Hello. This pull request adds a new injection to `injections.scm` for
Ruby language to highlight regular expressions. Thanks.

## Before

![CleanShot 2024-05-31 at 16 25
46@2x](https://github.com/zed-industries/zed/assets/1894248/8b88718e-8f13-4d61-b6f9-6a25b3ebcc57)

## After


![after](https://github.com/zed-industries/zed/assets/1894248/e11f6ec3-45c6-40f8-b6d9-ddbfd16a3331)

Release Notes:

- N/A

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
356fcec337Vitaly Slobodin committed on 5/31/2024, 4:02:59 PM· committed by GitHubparent08123a2
1 file changedLine totals unavailable