Remapper extensions (#984)

* Remapper extension API

* Fix build

* More work

* Fixes, thanks Gradle.

* Build fix

* Cleanup

src/main/java/net/fabricmc/loom/extension/LoomGradleExtensionImpl.java

@@ -247,6 +247,11 @@ public class LoomGradleExtensionImpl extends LoomGradleExtensionApiImpl implemen
 		return libraryProcessorFactories;
 	}
 
+	@Override
+	public ListProperty<RemapperExtensionHolder> getRemapperExtensions() {
+		return remapperExtensions;
+	}
+
 	@Override
 	protected <T extends IntermediateMappingsProvider> void configureIntermediateMappingsProviderInternal(T provider) {
 		provider.getMinecraftVersion().set(getProject().provider(() -> getMinecraftProvider().minecraftVersion()));