This PR fixes the snippets issue when a user loads multiple snippets for
the same language.
It also adds tests for the snippets register function.

**Note**
I've made the choice to differentiate snippets based on their body.
Thus, two extensions can provide the same snippet with the same prefix
but with different bodies.
WDYT ?

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #51014
Closes #52182 

Release Notes:

- Fixed different snippet extensions for the same language cancel each
other

---------

Co-authored-by: MrSubidubi <finn@zed.dev>