Motivation for this change

The Claws Mail fancy (webkit) plugin has been deprecated for security reasons, but there's a new litehtml-based HTML viewer plugin to take its place (as of Claws 3.17.4). This PR adds support for that plugin. All that's needed is the addition of the gumbo HTML parser (dependency of litehtml). The Claws release tarball includes a copy of litehtml, for better or for worse; litehtml isn't in Nixpkgs yet but maybe we should package it ourselves and de-embed Claws's copy.

I've tested this out myself and it renders HTML emails fine for me.

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)
  • The closure size for claws-mail goes from 1222376408 bytes to 1223641328 bytes (+1.21MiB).
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.