constexpr / consteval version of `pattern_formatter`?

[SpriteOvO]

Since C++20, common containers (std::string, std::vector, etc.) have constexpr supports (P0784). It might be possible to provide a pattern parser for pattern_formatter that parses literal constant pattern strings at compile-time.

At least for me, most of the use cases are constant pattern strings, so the compile-time parser could avoid the performance overhead and reduce attack vectors of dynamically parsing and building pattern instances at runtime.

@gabime Would you be interested in this feature? If you would be willing to merge it, I can find time in the next month to try and make it happen. I expect this will not introduce breaking changes.

[gabime]

Sounds good, as long as it stays simple and maintainable (I feel it might be complex though). I'd recommend moving it to a separate source file, or at least isolating it in a dedicated #ifdef block to avoid cluttering the main code.