Make sure we handle non file URIs properly

[davidanthoff]

Turns out that for example the github extension will open files that do not exist on disc via didOpen, with a URI that starts with pr:. So I think in general we probably should not assume that every URI we get has a corresponding file on disc.

I think we can fix this in the next iteration.

The general strategy is probably to remove the path property from Document, only have the _uri property there, and then make sure that in every place where we need a path, we first check whether the uri actually is a local file system uri.