Intent to prototype and ship: Support `document.execCommand` and related API in `<input>` and `<textarea>`

[Masayuki Nakano]

Summary: We support `document.execCommand` and related API only with
`HTMLEditor`, but accidentally it may work in `<input>` and `<textarea>`
if there is `HTMLEditor` instance in the document. I fix this to
support them without `HTMLEditor` like the other browsers.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1220696

Standard: Nothing

Platform coverage: all

DevTools bug: N/A

Other browsers:
https://wpt.fyi/results/editing/other/exec-command-with-text-editor.tentative.html?label=experimental&label=master&aligned

web-platform-tests:
https://searchfox.org/mozilla-central/source/testing/web-platform/tests/editing/other/exec-command-with-text-editor.tentative.html

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Masayuki Nakano <masa...@d-toybox.com>
Working on DOM, Events, editor and IME handling for Gecko