Allow for non-yarn mappings for MigrateMappingsTask (#141)

* Add getExtension utility method to AbstractLoomTask

* Support non-yarn mappings in MigrateMappingsTask

* Fix style

* Assume Yarn mappings if correct notation was not supplied

* Move load mappings block into its own its own method

Also added protected to getExtension on AbstractLoomTask

* Use Option instead of environment variables

* Assume V2 Yarn by default

* Some restructuring

All works the same.

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

@@ -58,7 +58,7 @@ public class GenIdeaProjectTask extends AbstractLoomTask {
 			return;
 		}
 
-		LoomGradleExtension extension = project.getExtensions().getByType(LoomGradleExtension.class);
+		LoomGradleExtension extension = getExtension();
 		project.getLogger().lifecycle(":Building idea workspace");
 
 		File file = project.file(project.getName() + ".iws");