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/src/main/java/eu/kanade/tachiyomi/network/JavaScriptEngine.kt

@@ -0,0 +1,26 @@
+package eu.kanade.tachiyomi.network
+
+import android.content.Context
+import app.cash.quickjs.QuickJs
+import eu.kanade.tachiyomi.util.lang.withIOContext
+
+/**
+ * Util for evaluating JavaScript in sources.
+ */
+class JavaScriptEngine(context: Context) {
+
+    /**
+     * Evaluate arbitrary JavaScript code and get the result as a primtive type
+     * (e.g., String, Int).
+     *
+     * @since extensions-lib 1.4
+     * @param script JavaScript to execute.
+     * @return Result of JavaScript code as a primitive type.
+     */
+    @Suppress("UNUSED")
+    suspend fun <T> evaluate(script: String): T = withIOContext {
+        QuickJs.create().use {
+            it.evaluate(script) as T
+        }
+    }
+}