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

Adds `editor: convert to base64` and `editor: convert from base64` to
the command palette. Both commands operate on the current selection, or
the word under the cursor when nothing is selected. The decode command
silently no-ops on invalid base64 input or non-UTF-8 decoded bytes,
consistent with how other convert commands handle untransformable input.

Release Notes:

- Added `editor: convert to base64` and `editor: convert from base64`
commands to the command palette

---------

Co-authored-by: Christopher Biscardi <chris@christopherbiscardi.com>