UFCS support

[moon-chilled]

The currently proposed grammar changes don't support UFCS (e.g. i"...".format). I suggest adding:

PostfixExpression:
    InterpolatedString . Identifier
    ...

[schveiguy]

I was coming here to say it's already covered, but you are right, since InterpolatedString doesn't appear as a PrimaryExpression, it's not covered.