[File format]sb2, sb and sjr

[Catrious]

Describe the file format
A Scratch 2 project (sb2), Scratch 1.0 project (sb), ScratchJr project (sjr)

Describe the expected behavior
sb2-sb
sb2-sb3
sb3-sb2
sb-sb2
sjr-sb3
etc.

Provide reference implementations
Same libraries as sb3 implementation libs, but some other might be needed for .sjr

Do you plan on implementing this yourself?
No

[Xeltalliv]

• sb3-sb2 - I actually got this converter working in convert.to.it, but my integration of pyodide just for this one converter is a mess. There should probably be a more centralized way to run python-based converters, before something like this is attempted again. Also that converter isn't very stable and fails on a lot of sb3 files. So at least for now, all that extra complexity for something so unrealiable seems not worth it.
• sb2-sb - The only existing converter is licensed under GPL-3.0, which is not compatible with GPL-2.0.
• sb2-sb3 - There are no existing standalone converters for this. Most people do the conversion by importing and exporting through the official editor. I think decoupling sb2 importer and sb3 exporter from the rest of the GUI will be quite difficult.
• sb-sb2 - The only existing converter is licensed under AGPL-3.0, which is not compatible with GPL-2.0. But it's old versions were licensed under BSD-3-Clause, which is compatible.