compat: beacon: re-enable the API but transform it to a no-op

from #40783 (comment 2854455)

• beacon.enabled very interesting

References

• https://developer.mozilla.org/en-US/docs/Web/API/Navigator/sendBeacon
• https://gitlab.com/librewolf-community/browser/source/-/issues/83 - an example of known site breakage (edit: with beacon disabled)
• https://searchfox.org/mozilla-central/source/dom/base/Navigator.cpp#1138-1179 - where to patch?

@tom seems trivial. Edit: IDK if the above would cause more breakage than it solves (edit: i.e vs just having beacon disabled)

@pierov why is this very interesting (I have my own thoughts, want to hear yours)?