Added fg plugin

[dabrowskiw]

Nothing fancy, just added the plugin.

Also, added a README.md in the plugin directory that is probably totally unnecessary for anyone with nix experience, but it's stuff I spent a while googling for so maybe this will save some time for other beginners like me.

[haennes]

This plugin has dependencies, which can be added to the path using an overlay like this:

        (final: prev: {
          yazi = prev.yazi.override {
            extraPackages = with prev; [ bat fzf ripgrep-all ];
          };
        })

will add a mechanism similar to this to #9

Edit: managed to get this working locally