Make fetch slash command visible in the command selector (#21302)

The `/fetch` command is naturally already accessible via the completion
menu when you type / in the assistant panel, but it wasn't on the "Add
Context" command selector. I think it should! It's a super nice/powerful
one, and I've seen folks not knowing it existed. Side-note: maybe, in
the near future, it'd be best to rename it to "`/web`, as that's an
easier name to parse and assume what it does.

<img width="700" alt="Screenshot 2024-11-28 at 16 52 07"
src="https://github.com/user-attachments/assets/37134e1c-c788-48ca-81ae-d7416e8c8706">

Release Notes:

- N/A

assets/icons/globe.svg

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