LibWeb: Implement BaseAudioContext.createGain

Author: https://github.com/shannonbooth
Commit: https://github.com/SerenityOS/serenity/commit/6466fca20a
Pull-request: https://github.com/SerenityOS/serenity/pull/24471

Tests/LibWeb/Text/input/WebAudio/GainNode.html

@@ -0,0 +1,23 @@
+<script src="../include.js"></script>
+<script>
+    function dumpAudioParam(param) {
+        println(`${param} current: ${param.value}, default: ${param.defaultValue}, min: ${param.minValue}, max: ${param.maxValue}, rate: ${param.automationRate}`);
+    }
+
+    test(() => {
+        const audioContext = new OfflineAudioContext(1, 5000, 44100);
+
+        const node = audioContext.createGain();
+
+        // Check prototype
+        let prototype = Object.getPrototypeOf(node);
+
+        while (prototype) {
+            println(prototype.constructor.name);
+            prototype = Object.getPrototypeOf(prototype);
+        }
+
+        // Audio Params
+        dumpAudioParam(node.gain);
+    });
+</script>