Support non-`isfs` virtual workspaces for client-side editing

[isc-bsaviano]

There are a bunch of places in our code where we check the scheme of a URI and use "file" to mean client-side editing. We should consider supporting any scheme other than isfs and isfs-readonly for client-side editing.

cc @gjsjohnmurray for comment

Original issue: intersystems/language-server#294

[isc-bsaviano]

I took a look at this and it's pretty complicated. To support this, we'd need to make our code that generates a URI for a document async. That code is used in a lot of places, including our exported API, which makes such a change very invasive. I'm not sure this is worth doing unless we also re-evaluate how we generate URIs.