Add import/export support for .excalidraw files

[septatrix]

Is your feature request related to a problem? Please describe.
Excalidraw is a popular web app for drawing diagrams etc. and sees increasing usage due to it simple usage (simply open the web page) and free/libre model. Now there are two problems:

• Sometimes I get sent excalidraw files which I would like to work on in rnote but cannot.
• Sometimes I started a sketch/diagram etc in rnote and would like to share it with other people to collaborate on. However, I do not want to force these people to use rnote and would like to simply provide them an rnote file.

Describe the solution you'd like
rnote should be able to import excalidraw files (in a best effort manner) and also export them.

Describe alternatives you've considered
excalidraw could do the same thing. However, as the rnote file format is not yet stabilized this is very unlikely. excalidraw is also the larger project and unlikely to want to support the format of every small drawing app.

Additional context
https://docs.excalidraw.com/docs/codebase/json-schema