Add support for statically named arguments with FMT_STRING

[alexezeder]

Possible with this PR and a compiler with non-type template arguments support:

fmt::format(FMT_STRING("{foo}{bar}"), "bar"_a = "bar", "foo"_a = "foo");
"{foo}{bar}"_format("bar"_a = "bar", "foo"_a = "foo");

Still impossible:

fmt::format(FMT_STRING("{foo}{bar}"), fmt::arg("bar", "bar"), fmt::arg("foo", "foo"));

/fmt/include/fmt/core.h:2310:39: error: static assertion failed: use statically named arguments with compile-time format strings: fmt::arg("name", value) -> "name"_a = value

[vitaut]

Thanks for the PR.