LibWeb: Stub MediaCapabilities IDL interface

Author: https://github.com/jamierocks
Commit: 4c5a176354
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1283
Reviewed-by: https://github.com/tcl3 ✅

Userland/Libraries/LibWeb/Forward.h

@@ -592,6 +592,10 @@ namespace Web::MathML {
 class MathMLElement;
 }
 
+namespace Web::MediaCapabilitiesAPI {
+class MediaCapabilities;
+}
+
 namespace Web::MimeSniff {
 class MimeType;
 }