Add --mixins flag to enable automatic mixin remapping

[LexManos]

Currently only supports refmaps.json
This removes the need for the runtime mixin.env.remapRefMap and mixin.env.refMapRemappingFile flags which are poorly documented and only support slow SRG string replacements. As well as only supporting SRG format. This can mess up if the mappings file contains anything that would get simple replaced when it shouldn't. Such as during obf->mcp renaming where fields are often called 'a'.
Example:
FD: A/a Client/field
a:Ljava/lang/String; -> field:Ljfieldvfield/lfieldang/String;
This is a concerned noted in its javadocs which shouldn't be an issue in versions that use SRG names. However I think it's better to address this during the remapping to prevent this issue in the first place.

Now there is a much larger task of actually bypassing the need for the AnnotationProcessor at all.
This would require going through all the possible paths that the AP outputs 'extra' mappings for. Which is a lot. I believe I have found all the cases in my work on Srg2Source. However it would be better if there was some official unit test set. Or if someone else could double check my work.

This Pr is mainly opened as a sanity check, I need someone who actually uses a dep with Mixins to test this. And as an opening to talk about the specifics of going full ham into finding the inheritance info ourselves for mixins.

Note: This adds a dependency on gson, which adds ~200kb(from 450kb) to the jar.

[PaintNinja]

Left some minor comments, how do I test this?

[PaintNinja]

You can use Jackson Core or Jackson Jr for a significantly smaller dep than GSON

[LexManos]

I looked into that jackson jr objects is 150kb and it requires core which is ~200kb. So no, gson is smaller.

[PaintNinja]

Suggested change

	if (this.owner != null)
	buf.append('L').append(this.owner).append(';');
	if (this.name != null)
	buf.append(this.name);
	if (this.quantifier != null)
	buf.append(this.quantifier);
	if (this.desc != null) {
	if (this.desc.charAt(0) != '(')
	buf.append(':');
	buf.append(this.desc);
	}
	if (this.tail != null)
	buf.append(this.tail);
	if (owner != null)
	buf.append('L').append(owner).append(';');
	if (name != null)
	buf.append(name);
	if (quantifier != null)
	buf.append(quantifier);
	if (desc != null) {
	if (desc.charAt(0) != '(')
	buf.append(':');
	buf.append(desc);
	}
	if (tail != null)
	buf.append(tail);

Omitting the this is a tad cleaner and avoids reading from untrusted final fields on the heap multiple times when the same values are already available on the stack as local variables.

[LexManos]

Last i looked it compiles to literally the same thing.

[LexManos]

I havent exposed the gradle side. But to test it you should just be able to run the jar with --input, --output, --mappings and --mixins specified. And then shove the resulting jar in your dev mods folder without renamer.enableMixins() and it should work.

We need a good SIMPLE mixin test mod. But I dont gave any and havent dug into mixin for a while to write one.