Add Fabric-Jar-Type to jar manifest.

Either 'classes' or 'sources', the idea is mod distribution platforms can check that sources jars are not uploaded accidentally.

Closes #939

src/main/java/net/fabricmc/loom/task/RemapJarTask.java

@@ -109,6 +109,8 @@ public abstract class RemapJarTask extends AbstractRemapJarTask {
 		// Make outputs reproducible by default
 		setReproducibleFileOrder(true);
 		setPreserveFileTimestamps(false);
+
+		getJarType().set("classes");
 	}
 
 	private void setupPreparationTask() {