Add nix support

[Nymphium]

This PR enables to develop in Nix environment and make free from opam command manipulations.
This Nix environment is reproducable and manages depexts of opam packages.

But currently dune build --root . in nix-shell fails with the following log:

File "bin/dune", line 2, characters 7-14:
2 |  (name satysfi)
           ^^^^^^^
Error: File unavailable:
/nix/store/iwmn1v600hn2yyx0111xfq4hkrh646zy-camlpdf-2.3.1-satysfi/lib/ocaml/4.13.1/site-lib/camlpdf/camlpdf.cmxa
File "tools/gencode/u.ml", line 34, characters 2-32:
34 |   Core_kernel.String.split_lines s
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error (alert deprecated): module Core_kernel
[since 2021-05] Use [Core] -- [Core_kernel] was renamed as [Core]

The versions in nix-shell is the following:

tool	version
ocaml	4.13.1
dune	3.0.3

These version is selected based on satysfi.opam.

[leque]

That compile error would be fixed by #320.

[na4zagin3]

I'm just curious and this is not directly related to this issue itself. I'm just interested in Nix. Is it possible to (hopefully automatically) convert OPAM repo https://github.com/na4zagin3/satyrographos-repo to a Nix repo (or something like that) so that Nix users can automatically use packages there (including past versions of SATySFi)?

[Nymphium]

Is it possible to (hopefully automatically) convert OPAM repo https://github.com/na4zagin3/satyrographos-repo to a Nix repo (or something like that) so that Nix users can automatically use packages there (including past versions of SATySFi)?

THIS PR forcuses to build and develop satysfi itself in Nix, so the answer is no. But it would be possible to build documentations with satysfi and each packages in Nix.

[Nymphium]

I found out that camlpdf can't build on Nix immediately due to lack of which command.