Deliberate conflict marker

[Earthcomputer]

Add an annotation (say, @Conflict), that can be added to an injector to mark it as conflicting with other injectors that target the same instruction. This tells MixinExtras to fail the injection in this case.

This has the following advantages:

• Ability to explicitly conflict with other injectors in cases where this is not currently possible (e.g. with injection points that can only be used on non-conflicting injectors)
• Clarifies the intention that the injector should be conflicting
  • This makes it easier for tools such as mcdev to know if a conflicting injector is intentional.
• Possibility for future expansion of scope (e.g. conflicting under certain conditions)