Add support for linking to local files

[lorefnon]

Resolves #122

[pomdtr]

Thanks for your contribution.

I think we can improve this a bit:

Here a url or type: https://raw.githubusercontent.com/.../example
excalidraw would not be handled properly.

Instead of relying on the file extension, maybe we can just open any url without scheme in vscode:

Ex: ./test.excalidraw or /tmp/readme.md

[lorefnon]

Instead of relying on the file extension, maybe we can just open any url without scheme in vscode:

This is what I originally did. However it opens excalidraw files as json. So I added special handling for .excalidraw extension. References to non excalidraw files do work.

I'll update this so absolute paths will be handled correctly. The Url handling can stay as it currently is, yes ?

[pomdtr]

I see, then it make sense. I'll play a bit with your branch locally to see if the linking behavior match my expectations