Add manifest with main class to built jar

Author: Lassebq

build.gradle

@@ -1,5 +1,6 @@
 plugins {
     id 'java'
+    id 'application'
     id 'eclipse'
 }
 
@@ -16,6 +17,12 @@ dependencies {
     implementation 'org.json:json:20220924'
 }
 
+jar {
+    manifest {
+        attributes('Main-Class': 'lbq.jsongen.Main')
+    }
+}
+
 artifacts {
     archives jar
 }