Add snippet commands (#18453)

Closes #17860
Closes #15403

Release Notes:

- Added `snippets: configure snippets` command to create and modify
snippets
- Added `snippets: open folder` command for opening the
`~/.config/zed/snippets` directory


https://github.com/user-attachments/assets/fd9e664c-44b1-49bf-87a8-42b9e516f12f