Enable UserActivation API in WebKit

https://bugs.webkit.org/show_bug.cgi?id=248192
rdar://102589438

Enables the UserActivation API in WebKit, which is available via
"navigator.userActivation".

The API is specified in the HTML spec:
https://html.spec.whatwg.org/multipage/interaction.html#the-useractivation-interface

This API is used to determine whether a user has interacted with the
page in a way that is considered to be "activation" (e.g. clicking a
button, or pressing a key).

Reviewed by Chris Dumez.

* Source/WTF/Scripts/Preferences/WebPreferencesExperimental.yaml:

Canonical link: https://commits.webkit.org/257214@main

Source/WTF/Scripts/Preferences/WebPreferencesExperimental.yaml

@@ -1774,7 +1774,7 @@ UserActivationAPIEnabled:
      WebKitLegacy:
        default: false
      WebKit:
-       default: false
+       default: true
      WebCore:
        default: false