Allow selecting multiple features via the URL `id=` param

[bhousel]

We currently support a url param id=osmid to select a single feature (see https://github.com/facebook/Rapid/blob/main/API.md?plain=1#L34)

It would be useful to support a list of ids to multiselect, like id=n1,n2,n3

I think this might have worked before , and I remember https://github.com/openstreetmap/iD/issues/2818 , but the code has changed a bunch since 2019, so it probably needs to be reimplemented.

Maybe related to #1406