Migrate the Enter and ShiftEnter plugins to beforeInput

[oleq]

Provide a description of the task

At this moment EnterObserver is used to detect Enter and Shift+Enter keystrokes. This can be handled via beforeInput insertParagraph and insertLineBreak event types, though, for instance:

editor.editing.view.document.on( 'beforeinput', ( evt, data ) => {
	const domEvent = data.domEvent;

	if ( domEvent.inputType === 'insertLineBreak' ) {
		editor.execute( 'shiftEnter' );

		evt.stop();
		data.preventDefault();
	}
} );

[oleq]

Features that listen to enter:

• BlockQuoteEditing,
• CodeBlockEditing,
• ListEditing,
• WidgetTypeAround

[oleq]

Idea: Split the logic inside EnterObserver to either use keydown or beforeInput.

[oleq]

Closed in #8176.