Intent to unship: Recursive call of Document.execCommand()

[Masayuki Nakano]

`Document.execCommand()` may cause firing DOM events and/or running some
observers. Therefore, web apps can call it recursively. However,
Chromium has already stopped supporting it (since 2014)[1][2].

And it'll be documented in execCommand spec[3].

This is testing in Nightly and early Beta[4] since 75 (Feb, 2020). So,
I'd like to drop the support in Release channel and late Beta too
starting from 82, but it's still under COVID-19 situation so that if you
have some concern, let me know.

Pref name: dom.document.exec_command.nested_calls_allowed
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1634262

[1]: https://bugs.chromium.org/p/chromium/issues/detail?id=338558
[2]:
https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/renderer/core/editing/commands/document_exec_command.cc;l=75;drc=301e5d079a1b4c29c5b17574d0470e6db7370acc
[3]: https://github.com/w3c/editing/issues/200#issuecomment-578097441
[4]: https://bugzilla.mozilla.org/show_bug.cgi?id=1611374

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