Restructure project

* use modules instead of source sets
* Kotlin JVM instead of multiplatform (for now) to make things simpler
* Warning: GameCell doesn't use mutableState anymore, this needs to be
           fixed for the UI to work!

domain/build.gradle.kts

@@ -0,0 +1,12 @@
+plugins {
+    alias(libs.plugins.kotlin.jvm)
+}
+
+kotlin {
+    jvmToolchain(11)
+
+    dependencies {
+        testImplementation(libs.kotlin.test)
+        testImplementation(libs.atrium)
+    }
+}