Reintroduce shadowJar support using external gradle plugin

Since the project now contains a dependency again (commons-cli), I need to be able to generate
jars with those dependencies packaged. (fat jars)
Partially reverts 28a461b8fc

build.gradle.kts

@@ -5,6 +5,8 @@ plugins {
     // Apply the application plugin to add support for building a CLI application in Java.
     application
     java
+
+    id ("com.github.johnrengelman.shadow") version "7.1.2"
 }
 
 repositories {