Update grammar.dd to support Elvis operator

[RazvanN7]

After (and if) [1] is merged.

[1] dlang/dmd#7242

[dlang-bot]

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[jacob-carlborg]

I think this [1] should be updated as well.

[1] https://dlang.org/spec/expression.html#conditional_expressions

[RazvanN7]

@jacob-carlborg There it is. I made a separate PR since it was easier to just select "Improve this page", then manually make a PR.

[andralex]

Closing pending DIP if anyone takes it.