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In fmt, there is a `FMT_CONSTEVAL` macro that resolves to `consteval` on "new enough" C++ (otherwise it doesn't do anything). For R 4.6+, the default C++ used is finally "new enough" (`__cplusplus > 201703L`).

This causes all `fmt::format()` calls to require a constant expression for `const char*` and `std::string&` input, which we are not currently doing via `fmt_arg`.

We have a runtime provided string, which must now be wrapped in `fmt::runtime()`, which is what we should have been doing all along.

Throwing an informative (and tested!) error when we can't find the package namespace

`#include <cpp11/R.hpp>` sets everything up the right way, and otherwise we get a warning from Rcpp which doesn't seem to end up being relevant for this use case

To avoid triggering a NOTE about usage of `Rf_findVarInFrame3()` on R 4.5, where technically we had the tools to avoid that