LibWeb: Implement TextEncoder.prototype.encoding

Author: https://github.com/linusg
Commit: f8387dea26
Pull-request: https://github.com/SerenityOS/serenity/pull/11231
Reviewed-by: https://github.com/alimpfard

Userland/Libraries/LibWeb/Encoding/TextEncoder.idl

@@ -5,5 +5,5 @@ interface TextEncoder {
     [NewObject] Uint8Array encode(optional USVString input = "");
     // TextEncoderEncodeIntoResult encodeInto(USVString source, [AllowShared] Uint8Array destination);
 
-    // readonly attribute DOMString encoding;
+    readonly attribute DOMString encoding;
 };