Support #![feature(extended_key_value_attributes)]

[jyn514]

Tracking issue: rust-lang/rust#78835. I hope to replace doc(include) with this, and it would be nice for it to have IDE support.

[jyn514]

Tangentially related: this has pretty bad error recovery in the parser, maybe it could be improved somehow?

[Veykril]

Parsing implemented in #8097, given #8092 I think this can be closed