Add sources jar (#4612)

* Add sources jar

* Update build.gradle

* Fix classes not found

* :core

* formatting

Co-authored-by: Anuken <arnukren@gmail.com>

build.gradle

@@ -318,6 +318,11 @@ project(":core"){
         }
     }
 
+    task sourcesJar(type: Jar, dependsOn: classes){
+        classifier = 'sources'
+        from sourceSets.main.allSource
+    }
+
     dependencies{
         compileJava.dependsOn(preGen)