Make fast_float::chars_format a scoped enumeration

[dalle]

Should fast_float::chars_format be changed into a scoped enumeration (enum class) to resemble std::chars_format?

I can do the work and PR, but I just wanted to check if you're okay with that change.

[lemire]

Yes, please see:

https://en.cppreference.com/w/cpp/utility/chars_format