Motivation for this change

(This is a sequel to the pull requests skktools: init at 1.3.3 #30778 and skk-dicts: init at 2017-10-26 #30960.)

libskk is a library that implements the kana-to-kanji conversion logic for the SKK Japanese input method.

The motivation for having libskk in nixpkgs comes from the desire to have
fcitx-skk in fcitx-engines, which depends on libskk. Further pull requests are planned for fcitx-skk (a fcitx engine).

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md