Move mod loader to MonoMod reorg/new HarmonyX

[BadMagic100]

Background

The newest version of MonoMod and HarmonyX support ARM processors; this is critical long-term for supporting Macs because Apple is not intending to keep Rosetta around forever. We had initially planned to help with the update to BepInEx 7 to make this happen, but then we got scope creeped and stonewalled, so we need another solution. This could be a fork of BepInEx or potentially a completely custom solution

Requirements

• ARM compatibility. This is solved by the newer versions of MonoMod/HarmonyX but worth stating explicitly
• No replacement of game files at any point; it should be trivial to launch the game vanilla by changing config only (for Mac and Linux, this looks like running the original executable rather than the wrapper script).
• Binary compatibility with the BepInEx 5 API. This means that plugins and preloader plugins should work, config API should work, Harmony should work, etc. We know that even people in the discord do not follow our guidance accurately so we should be accommodating for the majority of mods just to work even if they didn't pay close attention to what they're supposed to do. Even for HK, we are now starting to see the usage of BepInEx crop up and cause problems, so we cannot ignore it and pretend things will be fine because Big Mod X uses our approach.
• Nice to have - plugins using monomod directly are not broken; ie there is a compatibility layer to bring people to reorg with no effort
• Nice to have - preloader plugins can be written in modern-er .net

Implementation Idea

Just fork BepInEx v5-lts branch and do the monomod/harmony update. It does mean maintaining a fork but it's a super easy approach in theory.

[BadMagic100]

Also in scope of this: given that everyone and their dogs have written a dependency on BepInEx-BepInExPack_Silksong, what needs to happen to replace this? We do know that thunderstore gives some special treatment to bepinex though because of universal plugins that don't depend on game specific bepinex packs

Maybe the answer lies here somewhere? https://github.com/thunderstore-io/ecosystem-schema/blob/master/games/misc/modloader-packages.yml

[dpinela]

If/when we make our own installer (as planned) we could have it replace the dependency automatically without requiring previous manifests to change. Given that the replacement BepInEx would be binary-compatible with existing mods, existing installers would keep working, but the new one would have the upside of supporting ARM macOS natively.

[BadMagic100]

without requiring previous manifests to change

Bepinex pack is already special cased by installers so I think this may not actually be a problem. You can see for example the packages for monodetour depend on plain bepinex, but when installed in silksong they will install the silksong bepinex pack instead. I believe same will apply if we do our own replacement