An attribute for editable denoting what enter mode it has

[Olek Nowodziński]

Use case:

When focusing a widget, editor.activeEnterMode updates accordingly to ACF rules in nested editables. If the widget remains focused and magicline inserts a focus space outside of a widget, it will follow the enter behavior imposed by nested editable which is not necessarily the same as editor's one. Yet, if the insertion was within widget's editable, that enter mode would be correct.

To resolve that issue, considering the fact that magicline shouldn't deal with widgets API, an attribute must be established that denotes the enter mode inside of editable element. With such attribute, magicline would correctly insert either P or BR.

Blocker of #10828.

[Piotrek Koszuliński]

Part of #10881.

[Olek Nowodziński]

Introduced in #10881.