fix mixins not working on forge

forge/build.gradle

@@ -10,7 +10,8 @@ plugins {
 }
 
 mixin {
-    config("${mod_id}.mixins.json")
+    add sourceSets.main, "${mod_id}.refmap.json"
+    config "${mod_id}.mixins.json"
 }
 
 minecraft {
@@ -60,6 +61,15 @@ dependencies {
     implementation('net.sf.jopt-simple:jopt-simple:5.0.4') { version { strictly '5.0.4' } }
 }
 
+// for some reason mixin plugin does not add 'MixinConfigs' to MANIFEST.MF so we do it manually
+jar {
+    manifest {
+        attributes([
+                'MixinConfigs': "${mod_id}.mixins.json"
+        ])
+    }
+}
+
 sourceSets.each {
     def dir = layout.buildDirectory.dir("sourcesSets/$it.name")
     it.output.resourcesDir = dir