Intent to ship: selectionchange for input/textarea

[Kagami Rosylight]

Summary: selectionchange for text controls has been behind the flag `dom.select_events.textcontrols.enabled`, while the behaviour was different from Chrome’s as the events fired at each element instead of `document`. I intend to modify the behaviour to make the events fire at `document` and thus follow Chrome’s behaviour.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1648944 and https://bugzilla.mozilla.org/show_bug.cgi?id=1309626

Standard: https://github.com/w3c/selection-api/issues/53

Platform coverage: All

Preference: Enabled by default, but still controlled by `dom.select_events.textcontrols.selectionchange.enabled`. Note that the flag is changed to enable `selectstart` separately in the future.

Other browsers:
• Chrome: Shipped
• Safari: Shipped

web-platform-tests: There is no wpt coverage yet.
• Blocked by test_driver not allowing intermediate mouse state https://github.com/web-platform-tests/wpt/issues/24944

Secure contexts: No, as other browsers already shipped this without restriction

Is this feature enabled by default in sandboxed iframes?: Yes

[Kagami Rosylight]