Intent to Ship: inputmode attribute on Firefox Desktop

[Makoto Kato]

inputmode attribute is a hint to adjust software keyboard layout
(numeric, email and etc). Although this is already shipped on GeckoView,
this will be shipped at Firefox Desktop too.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1205133

Link to standard:
https://html.spec.whatwg.org/dev/interaction.html#input-modalities:-the-inputmode-attribute

Platform coverage: All. But macOs doesn't have a virtual keyboard.

Preference: This is enabled/disabled via dom.forms.inputmode

Devtools support: N/A. DevTools doesn't emulate software keyboard.

Other browsers: Chrome 80+ and Safari 12.2+

web-platform-tests: reflection test for HTMLElement is
https://searchfox.org/mozilla-central/source/testing/web-platform/tests/html/dom/reflection-misc.html.

Restricted to secure contexts: No