LibWeb: Implement TextEncoderStream

Required by the server-side rendering mode of React Router, used by
https://chatgpt.com/

Note that the imported tests do not have the worker variants to prevent
freezing on macOS.

Libraries/LibWeb/Encoding/TextEncoderStream.idl

@@ -0,0 +1,11 @@
+#import <Encoding/TextEncoder.idl>
+#import <Streams/GenericTransformStream.idl>
+
+// https://encoding.spec.whatwg.org/#textencoderstream
+[Exposed=*]
+interface TextEncoderStream {
+    constructor();
+};
+
+TextEncoderStream includes TextEncoderCommon;
+TextEncoderStream includes GenericTransformStream;