Make fmt::as_identifiers a struct instead of a variable

[avikivity]

The annotation was a constant of an empty type, which left no room for options. Make the annotation type itself, fmt::as_identifiers, the thing users write, so it is now spelled

enum class [[=fmt::as_identifiers()]] color { red, green, blue };

and options can later be added as constructor arguments.

[avikivity]

Note: this logically conflicts with #4899. If you want both (and you should), merge one and I'll refresh the other.

[avikivity]

v2:

• rebased past e589a16
• fixed clang-format error

[avikivity]

v3:

• rebased past bb1bced

[avikivity]

v4:

• rebased past fa9590f

[vitaut]

What kind of options do you envision?

[avikivity]

Let's say one's coding style is scary ENUM_OPTION_1 but one doesn't want to inflict it on one's users. One then writes [[=as_identifiers{.case = fmt::enum_options::lower}]] to show one's appreciation to one's coworkers, who would then likely respond by sending one some flowers and chocolate.

[avikivity]

I also expect something like [[=fmt::format_as("alternative")]] when a particular identifier isn't suitable, but that isn't on the enum type itself.

[avikivity]

Also: [[=fmt::as_identifiers{.with_numerical_value = true}]] useful when the enum represents something from a spec rather than just a list of stuff.