Identify nestedElement fields even if their content expression is overridden

[rhystmills]

What does this change?

The workaround for the text element join after deleting a non-text element between two text elements relied on nestedElement fields having the content expression element+.

This is flawed, because the fields can have their content expression overridden - in fact we will need to override it to something like nonKeyTakeawaysElement+ in order to prevent a key takeaway element being pasted into another a key takeaway element.

This PR uses the stable "nested-element-field" group that should appear in nestedFields as an identifier for our workaround to use. In the long run we'll want to fix the route cause of the nestedElement fieldView update bug that caused the join issue, rather than relying on this workaround.

n.b. Groups are modelled in a string of space-separated group strings rather than as an array of strings.

How to test

If you're feeling diligent, use this version of prosemirror-elements locally in Composer via yalc.

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