Gradle Kotlin DSL (#2634)

* Ease migration to Gradle Kotlin DSL by changing quotes, function calls and plugin definitions

* Migrate build scripts to Gradle Kotlin DSL

buildSrc/src/main/kotlin/BuildConfig.kt

@@ -0,0 +1,13 @@
+package com.unciv.build
+
+object BuildConfig {
+    const val kotlinVersion = "1.3.71"
+    const val appName = "Unciv"
+    const val appCodeNumber = 428
+    const val appVersion = "3.8.9"
+
+    const val gdxVersion = "1.9.10"
+    const val roboVMVersion = "2.3.1"
+    const val ashleyVersion = "1.7.0"
+    const val aiVersion = "1.8.0"
+}