Add JavaScriptEngine abstraction to extensions-lib (#8155)

This allows us to swap out the implementation in the future and on different platforms
without major changes to the extensions themselves.

core/build.gradle.kts

@@ -43,4 +43,7 @@ dependencies {
     api(libs.preferencektx)
 
     implementation(androidx.corektx)
+
+    // JavaScript engine
+    implementation(libs.bundles.js.engine)
 }