Release version 2.0.0 and add test-app setup.

Updated the version from 2.0.0-SNAPSHOT to 2.0.0 in `build.gradle.kts` for release readiness. Added a new `test-app` project including Gradle build files, scripts, and configuration for Kotlin multiplatform with JS and JVM targets to enable testing and application runtime.

.gitignore

@@ -45,3 +45,9 @@ bin/
 .kotlin
 
 gradle.properties
+
+test-app/data
+test-app/kotlin-js-store
+test-app/build
+test-app/.gradle
+test-app/.kotlin