Support editorClassName

[kj800x]

We'd like to be able to target the draft-extend-editor in our styling, but are concerned about styling a class name that we didn't provide. We would like the draft extend editor to take in an editorClassName prop which is applied to the draft-extend-editor node, so that we can provide a className to style.

cc @bent0b0x

See LiveMessagesUI pull #2847 on the HubSpot internal git for futher context.