Allow Quilt mods to specify their Mixin compat version

[EnnuiL]

With PR #448 merged and out of our way, we can finally ponder the question: Quilt Loader formerly assumed that all Quilt mods would have the latest Mixin compat version (which at the time was 0.10.0), which introduced a problem when the 0.14.0 compat version was introduced with Fabric Loader 0.16.0.

Fabric Loader handles this by having this tied with dependencies on a specific minimum Fabric Loader version, but this is prone to mistakes happening and suddenly dooming yourself to an older Mixin compat version (with 0.9.0 being a particularly nasty one due to broken locals).
Therefore, we believe that for Quilt mods? an explicit (and opt-in) field encouraged by the Quilt Template Mod would be a better solution in order to handle this case properly (after all? you have to change JAVA_17 to JAVA_21 on the .mixins.json and despite the need of a newer QLoader? It doesn't need a change of dependencies)

Currently, all Quilt mods are pinned to the 0.10.0 mixin compat version so the FLoader 0.16.2 sync could move on (goddamn y'all took a long while to merge), but now? y'all are free to shred yourselves apart :p