Cache _Fmt_codec<char, false>

[cpplearner]

This is an alternative to #2596.

Fixes #2594

I thought of simply returning a global _Fmt_codec<char> object in _Get_fmt_codec<char>(), but I think this might make std::format unusable in initialization of variable template instances. (The standard says that such initializations are unordered with other initializations.)

[barcharcraz]

This looks great to me, though someone who understands more about object lifetime should look at the caching component.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for improving <format> performance with these codepages! 🚀 ⚡ 🚅