Add devshells

[xaltsc]

This is an attempt at solving #539.

The current commit is an extremely crude approximation of what is needed. PLEASE DO NOT MERGE.

There's a competing effort in PR #538.

Blocked by #528

[xaltsc]

So, I'm trying to use nixpkgs''s mpv` libs as requested by @andrewrabert.

$JFN_MPV_INCLUDE_DIR has been set to various values, however, it's still impossible to find client.h.

I'm getting the following error when running just clippy:

  --- stderr
  Error: "could not locate mpv/client.h in any of: [\"third_party/mpv/include\"]\nSet JFN_MPV_INCLUDE_DIR, EXTERNAL_MPV_DIR, or install libmpv via pkg-config."

So it doesn't look like JFN_MPV_INCLUDE_DIR is considered at all.

[xaltsc]

Also, when running just clippy with CEF_PATH set (to the wrong one, but that's not the issue), I get

  CEF_PATH is invalid (File I/O error: failed to open file `/nix/store/v4lvgffqpc359ya581qw0p6fcralbpfk-cef-binary-149.0.4/archive.json`: No such file or directory (os error 2)),

No such archive.json exist. I assume it's something that exist in the downloaded tarball which is the scenario that clippy expects. But build works without it.

[xaltsc]

Also, just build fetches mpv and CEF, which is unnecessary, and even tries to build them (afaict, libplacebo is not a dependency of jfnd itself).

[xaltsc]

I don't think I can work on this PR further without taking a maximalist approach (which I refuse to).

Build methods needs to be patched before I can advance.

[andrewrabert]

@xaltsc please combine the nix PR's and I'll marge asap