Motivation for this change

(This is a sequel to the pull request skktools: init at 1.3.3 #30778.)

In this package skk-dicts, we collect some standard dictionaries that can be used
by various SKK Japanese input methods.

The motivation for having skk-dicts in nixpkgs comes from the desire to have
fcitx-skk in fcitx-engines, which depends on libskk
( https://github.com/ueno/libskk ), which in turn depends on one or more SKK dictionaries.

We could have fetched SKK dictionaries from within libskk package, but we've
decided to factor out the dictionaries as a standalone package, as they can
in principle be utilized by other SKK input methods, such as ibus-skk, scim-skk etc.

Further pull requests are planned for libskk and fcitx-skk (a fcitx engine).

Note:
The SKK openlab site ( http://openlab.ring.gr.jp/skk/dic.html ) used to be the primary
distribution site for these dictionary files, but it does not appear to be actively maintained
any longer. Furthermore, the files on the openlab site are not version controlled.
We therefore fetch dictionary files from a (semi-official?) fork on github
( https://github.com/skk-dev/dict ), which at least is version controlled.

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.