"Prefix unused value with underscore" code fix should not appear on double ticked names

[vasily-kirichenko]

[forki]

Lol

[forki]

Speaking more generally: we should filter suggestions that lead to invalid identifiers.

[vasily-kirichenko]

I don't think it's a good strategy, it would hide bugs in suggestions.

[forki]

I'm fine with handling these incrementally whenever we see one appearing

[vasily-kirichenko]

Fixed in #2424