LibWeb: Add MediaList

This is a list of MediaQuery objects. Not to be confused with
`MediaQueryList`, which is concerned with firing events when a media
query's match-state changes.

Userland/Libraries/LibWeb/CSS/MediaList.idl

@@ -0,0 +1,7 @@
+interface MediaList {
+    stringifier attribute [LegacyNullToEmptyString] CSSOMString mediaText;
+    readonly attribute unsigned long length;
+    getter CSSOMString? item(unsigned long index);
+    undefined appendMedium(CSSOMString medium);
+    undefined deleteMedium(CSSOMString medium);
+};