Add speech synthesis support

[vitaut]

Add speech synthesis support to {fmt}.

For example:

#include <fmt/os.h>

int main() {
  fmt::say(
    R"SHANTY(
    There [[emph +]] once was a pen that [[emph +]] put to [[emph +]] paper,
    It [[emph -]] was [[emph +]] gonna be written [[emph +]] sooner or later,
    Ctrl-S went down, the proposal sent out,
    Vote [[emph +]] down my committee vote [[emph +]] down.

    [[emph +]] Soon, may the Unicode come,
    So [[emph +]] we can write aleph and [[emph +]] xĭ and [[emph +]] từng,
    One day when '23 is done,
    We'll take our bom and go.)SHANTY");
}

Output:

say3.mov

[tigerw]

The best kind of feature creep. 3D text render output when

[vitaut]

3D text render output when

PRs are welcome (not).

[rcombs]

In all seriousness, I don't think this is a great idea to have in the library as-is, since it might not be obvious to users that this isn't safe to pass arbitrary input to, nor that it's highly non-portable (say only exists on macOS).

It'd be safer to at least use FILE* f = popen("say", "w"); and fmt::print(f, format_str, args…);.