Add legacy forge support

[MsRandom]

Legacy meaning fg2 pipelines, meaning pre-1.12.2

Relevant implementation info in the userdev jar:

• classes.jar is an inject
• sources.zip is sources for generated patched jar(+ decompilation results?)
• devbinpatches.pack.lzma is binpatches, meant to be used with the standard binpatcher
• dev.json is the fg2 userdev config
• access transformer included
• mcp config is de.oceanlabs.mcp.mcp:$mc-version:(srg or csrg), includes srg mappings
• exceptions are in both the userdev jar and mcp config