Switch to our own Gradle plugin for vanilla Minecraft

I didn't make a new years resolution to stop writing build tooling, but
maybe I should have.

This replaces our use of VanillaGradle with a new project,
VanillaExtract. This offers a couple of useful features for multi-loader
dev, including Parchment and Unpick support, both of which we now use in
CC:T.

projects/common/build.gradle.kts

@@ -22,6 +22,14 @@ configurations {
     register("cctJavadoc")
 }
 
+repositories {
+    maven("https://maven.minecraftforge.net/") {
+        content {
+            includeModule("org.spongepowered", "mixin")
+        }
+    }
+}
+
 dependencies {
     // Pull in our other projects. See comments in MinecraftConfigurations on this nastiness.
     implementation(project(":core"))