The onRichTextPaste callback does not get called when pasting Delta internally

[EchoEllet]

This callback provides the isExternal parameter, but it will not be called when pasting a Delta internally, so the isExternal is always true. It will need to be called when the user pastes a Delta into the editor clipboard (internal).

• Related Add paste options (bool plainTextOnly and a callback to view/modify a delta prior to pasting) #2350