Add FernFlower for Mixin Debugging (#94)

* Add FernFlower for mixin debugging

* Fix mistake

* Add condition

Co-authored-by: Jason Mitchell <mitchej+github@gmail.com>

build.gradle

@@ -330,6 +330,12 @@ repositories {
             name = "GTNH Maven"
             url = "http://jenkins.usrv.eu:8081/nexus/content/groups/public/"
         }
+        if (usesMixinDebug.toBoolean()) {
+            maven {
+                name = "Fabric Maven"
+                url = "https://maven.fabricmc.net/"
+            }
+        }
     }
 }
 
@@ -339,6 +345,9 @@ dependencies {
         annotationProcessor('com.google.guava:guava:24.1.1-jre')
         annotationProcessor('com.google.code.gson:gson:2.8.6')
         annotationProcessor('com.gtnewhorizon:gtnhmixins:2.1.3:processor')
+        if (usesMixinDebug.toBoolean()) {
+            runtimeOnly('org.jetbrains:intellij-fernflower:1.2.1.16')
+        }
     }
     if (usesMixins.toBoolean() || forceEnableMixins.toBoolean()) {
         compile('com.gtnewhorizon:gtnhmixins:2.1.3')