Add Shadow plugin and configure shadowJar task

plugin/build.gradle.kts

@@ -1,6 +1,7 @@
 plugins {
     id("java-library")
     id("net.minecrell.plugin-yml.bukkit") version "0.6.0" // Plugin for generating plugin.yml
+    id("com.github.johnrengelman.shadow") version "8.1.1"
 }
 
 dependencies {
@@ -21,3 +22,11 @@ bukkit {
     description = project.description
     version = project.version.toString()
 }
+
+tasks.shadowJar {
+    archiveClassifier.set("")
+}
+
+tasks.build {
+    dependsOn(tasks.shadowJar)
+}