LibWeb: Define PerformanceEventTiming

https://www.w3.org/TR/event-timing/#sec-performance-event-timing

Add idl, header and stubs for PerformanceEventTiming interface.

Two missing `PerformanceEntry` types that have come up in issues
are the `first-input` and the `event` entryTypes. Those are both
this.

Also, because both of those are this same interface, the static
methods from the parent class are difficult to implement because
of instance-specific details. Might either need subclasses or to
edit the parent and also everything that inherits from it :/

Userland/Libraries/LibWeb/Forward.h

@@ -307,6 +307,10 @@ namespace Web::EntriesAPI {
 class FileSystemEntry;
 }
 
+namespace Web::EventTiming {
+class PerformanceEventTiming;
+}
+
 namespace Web::Fetch {
 class BodyMixin;
 class Headers;