Add initial Quilt submodule config

- Added missing `gradle.properties` for the `:quilt` module
- Added `include("quilt")` to `settings.gradle`

Not sure what this setup really does beyond setting Loom's platform to Quilt.
This doesn't currently build — possibly due to:
A) Missing support, or
B) Me doing something wrong.

This branch might be dead, but at least this stuff is tracked now.

Best regards,
my future self (if I ever come back to this)

quilt/gradle.properties

@@ -0,0 +1 @@
+loom.platform=quilt