Enable linkcheck for Excalidraw rooms

[adamtheturtle]

Excalidraw uses the concept of "Rooms" for shared diagrams.

Each room has an identifier, and the URL includes #.

For example: https://excalidraw.com/#room=48f545e2b526bb9f4a9a,e1RQwkUUeH-AZ4Pyb4bwKw.

The linkcheck builder gives an error like: Anchor 'room=XXX' not found.
However, the room is not really an anchor.

Describe the solution you'd like

I would like a way to say "For this link, please check that the room is available, not just that excalidraw.com is available.