Add `/whois` command

[bbhtt]

Your use case

What would you like to do?

Open the userinfo page for an account quickly.

Why would you like to do it?

To see user details:

How would you like to achieve it?

A slash command /whois <mx-id>

Have you considered any alternatives?

Search the account in a room list and click from there? But the user and the person searching may not have a shared room and for large rooms the entire memberlist takes time to load.

Use a matrix.to link to the profile (not sure if it opens the profile)

Additional context

Element web has this implemented:

Are you willing to provide a PR?

No