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/CMakeLists.txt

@@ -219,6 +219,7 @@ set(SOURCES
     Encoding/TextDecoder.cpp
     Encoding/TextEncoder.cpp
     EntriesAPI/FileSystemEntry.cpp
+    EventTiming/PerformanceEventTiming.cpp
     Fetch/Body.cpp
     Fetch/BodyInit.cpp
     Fetch/Enums.cpp