Fix non_upper_case_globals lint in some cases

[WorldSEnder]

This was never reported as a bug, but mentioned on the discord, that sometimes the macro emits warnings that can be ignored with #![allow(non_upper_case_globals)]. I believe the culprit in the situation is the emitted const _: when parts of the style AST can be constructed at compile time.

I've added a small comment on the usage of Span::mixed_site() for future reference too, since I had to look that up when editing.

[WorldSEnder]

MSRV test failure seems to come from a downstream package, i.e. toml_datetime requiring 1.67 compared to the tested 1.65.