This commit adds a new string formatting system to replace uses of
`SDL_snprintf` and string concatenation.
Making our own string formatting system has been briefly discussed
during the review of the localization branch, and on the VVVVVV
Discord. It's inspired by Python's format strings, but simpler.
This is primarily to benefit localization - strings will be easier to
understand (`Now using %s Tileset` → `Now using {area} Tileset`,
`"%s remain"` → `"{n_crewmates|wordy} remain"`), translators can change
the word order for their language's grammar (`%1$s` is a POSIX
extension), and this system is also less error-prone (making the format
string not align with the actual arguments won't result in a crash or
UB).
It also integrates our needs better - particularly the "wordy" numbers
without having to have a `help.number_words(n).c_str()` at the
callsite, translators can opt in and out of wordy numbers per string,
and this should also make it easier to solve #859.
This commit adds the formatting system itself, and changes one
`SDL_snprintf` in the code to use it as a small demo (the rest should
probably be done in the localization branch to avoid more unneeded
work).
The system is described in full detail in VFormat.h and in the pull
request description.
