Ship a SDK with elfldr releases

[modeco80]

Ditto.
This SDK would provide:

• LibUtils headers
• Runtime headers
• Game API headers
• ERL ABI headers
• LibUtils-ERL
• Runtime library
• An external/SDK-shipped build system for patch/gamehook authors to build ERLs
• A sample ERL for making a patch ERL and a gamehook ERL

Needs to be completed before #4 is closed, as it augments it (I mean, I don't want to have to force everyone to do in-tree builds, at least.)

Potentional maybe: a elfldr-sdk repository people can vendor to, that Actions pushes to?

[modeco80]

sort of flux closed by 4686776, but the "SDK" isn't ready yet (and I probably need to get to work on making the libraries have cmake import stuff, so that the SDK can be used pretty much exactly like building chs in tree)