Port to 1.21.1 neoforge

I wanted to play with Tetra but I use 1.21.1 neoforge, so I ported it. So far in my testing everything seems to be working (all UIs, item modification, enchanting, behavior of modified items, toolbelts) correctly but I haven't tested absolutely everything. Full disclosure I did this by directing gpt-5.4-xhigh so feel free to close if you don't want to review AI generated stuff. Github also doesn't allow creating a PR against a new branch, so this should probably live in a new branch if you want it, not merged into 1.20. PR to mutil as well https://github.com/mickelus/mutil/pull/17