aspellWithDicts: create derivation with aspell and selected dictionaries

Currently, `aspell` checks the active profiles for dictionaries. While
this may be convenient, it does not work with `nix-shell` and it doesn't
allow any isolation.

This commit adds the possibility to use composition by creating a
derivation with `symlinkJoin` that contains all the chosen dictionaries,
and another derivation that wraps the executables linking to the dictionaries.

Nix example:

my_aspell = aspellWithDicts(ps: with ps; [ en nl ])

`nix-shell` example:

nix-shell -p 'aspellWithDicts(ps: with ps; [ en nl ])'