LibWeb: Add the TextEncoder interface

This is from the Encoding Standard (https://encoding.spec.whatwg.org),
and therefore gets its own namespace and subdirectory within LibWeb :^)

Userland/Libraries/LibWeb/Encoding/TextEncoder.idl

@@ -0,0 +1,9 @@
+[Exposed=(Window,Worker)]
+interface TextEncoder {
+    constructor();
+
+    // [NewObject] Uint8Array encode(optional USVString input = "");
+    // TextEncoderEncodeIntoResult encodeInto(USVString source, [AllowShared] Uint8Array destination);
+
+    // readonly attribute DOMString encoding;
+};