Add @DefinitionSource annotations

Allows parameterized tests to use Item, Npc, and Object definitions
as @ArgumentSources.

This commit also adds support for using @ItemDefinitions etc on
properties as well as functions.

game/plugin-testing/build.gradle

@@ -19,4 +19,10 @@ dependencies {
     api group: 'com.willowtreeapps.assertk', name: 'assertk', version: assertkVersion
 
     implementation group: 'org.powermock', name: 'powermock-module-junit4', version: powermockVersion
+}
+
+tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all {
+    kotlinOptions {
+        jvmTarget = "1.8"
+    }
 }