allow customizing mixinConfigRefmap

[WaitingIdly]

changes in this PR:

• adds a new gradle.properties entry, mixinConfigRefmap, which defaults to an empty string.
• sets the new mixinConfigRefmap to mixins.${project.modId}.refmap.json if unset (doesnt exist or is an empty string)
• uses mixinConfigRefmap in the places where the refmap is targeted.

this allows users to have a custom refmap file instead of requiring it to be mixins.${project.modId}.refmap.json. currently, if a user named this something that isnt a json file (for example, silly.png), the refmap file would still work correctly (this was tested, it injects mixins properly). however, due to my strong personal conviction that this should not be done, there is a verbal instruction that it must be a json file in the comment.

[ALongStringOfNumbers]

Hm. Is there any reason why you could not just set generateMixinConfig to false, and then add in your own config? We do this in GT, so we can have a refmap per mod that we are targeting, for organization. I am wondering if it might be easier to have users who want custom refmaps just create their refmap and turn the config to generate a refmap off, instead of going through another config to generate a custom refmap.