Motivation for this change

Part of @Mic92 and my plan to package Mozilla TTS.

This step includes phonemizer, a library to convert text to phonemes.

It supports multiple backends, like espeak and festival. Unfortunately we couldn't get the festival backend to work, so we didn't include it.

E           RuntimeError: Command "/nix/store/3jb8rhr9ls6lwmrsahnwcq4v53kv3aw3-festival-2.5.0/bin/festival -b /build/tmpyi73uuuh" returned exit status 255, output is:
E           -=-=-=-=-=- EST Error -=-=-=-=-=-
E           {FND} Feature Token_Method not defined
E
E           -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
E           closing a file left open: /build/tmpyi73uuuh

According to https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=687822 we are likely missing two folders, voices and dict. But for us the festival integration is out of scope for now. The espeak backend works and it's what we are using.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.