Build natively to 1.21/NeoForge by using Architectury Loom

[zbx1425]

This PR attempts to apply minimal changes to achieve building natively to a NeoForge target, which might appeal to those who don't prefer getting on board a full Sinytra Connector setup just for Continuity or couldn't do that due to compatibility issue.

• Needed FFAPI components can be JiJ-ed to remove one more dependency (though maybe this can be seen as some overkill, or there might be potential JiJ version conflict issue that needs investigation)
• Or we can just remove requirement for Sinytra Connector and still require player to separately install FFAPI.