Deleting composable characters

[maddyjoe]

As a young person using emojis in a project
I want to be able to press delete/backspace
And have the whole emoji be deleted

Background
ATM CodeMirror will only partially delete an emoji where it is made up of several characters. The patch they rolled out does not fix the issue (as it breaks other stuff that we want to work for e.g. where characters are used in translated languages).
This was the case when we looked at this under ticket #213
Further details:
codemirror/dev#516
We may need to revisit this to see if/when CodeMirror post an update.