Add getBaseMappedJar to make it easier to get the pre-transformed game jar.

src/main/java/net/fabricmc/loom/configuration/providers/minecraft/MinecraftMappedProvider.java

@@ -210,6 +210,10 @@ public class MinecraftMappedProvider extends DependencyProvider {
 		return minecraftMappedJar;
 	}
 
+	public final File getBaseMappedJar() {
+		return minecraftMappedJar;
+	}
+
 	public File getUnpickedJar() {
 		return new File(getExtension().getMappingsProvider().mappingsWorkingDir().toFile(), "minecraft-unpicked.jar");
 	}