Intent to prototype: enterkeyhint attribute

[Makoto Kato]

enterkeyhint is a HTML element attribute that is for software keyboard
layout. Software keyboard can change "ENTER" key cap to others such as
"GO", "SEND" and etc. Firefox OS has "moz_action" attribute to change
this, but this enterkeyhint is the standardized version of this.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1490661

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

Platform coverage: All. Except to Android, no OS API to change ENTER
key cap to others.

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

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

Other browsers: Chrome 80 and Safari 13.1

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

-- Makoto