Intent to prototype and ship: HTMLInputElement showPicker()

[Tom Schuster]

Summary: showPicker enables opening the color and file picker (similar to the click method) programmatically, but also for the first time date/time pickers.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1745005

Specification: https://html.spec.whatwg.org/multipage/input.html#dom-input-showpicker

Platform coverage: All (date/time support on Android is still todo, but soon)

Preference: dom.input.showPicker

Other browsers:

- Chrome shipped in 99

- Safari landed: https://bugs.webkit.org/show_bug.cgi?id=237192

web-platform-tests:

- Yes, but we also have our own tests for the actual UI. See the bug