implement support for rooms.

[eric-schleicher]

Is your feature request related to a problem? Please describe.

I use (almost exclusively) rooms to share data and create persistent unique urls for my excalidraw boards. this means that i'm always off the main excalidraw root (/). e.g. https://excalidraw.com/#room=34974016b3ac57acc166,g3ve6SzKsqf_V78APaVAIQ

Describe the solution you'd like
It would be cool if the plugin worked on the rooms.

Describe alternatives you've considered
I would be willing to attempt making the changes, but wanted to stop first and see if it's trivial, or something more involved.

Additional context
Add any other context or screenshots about the feature request here.