Add initial flake.nix

Damien Cassourequested to merge
DamienCassou/rycee-nur-expressions:flake into master

NUR is looking for a maintainer because the current maintainer believes that flakes have replaced the use for NUR. I believe this can only work if each NUR repository has a flake.nix file. This PR introduces an initial version of it.

For now, only firefox addons are exported because this is the only thing I need. I can add more exports if you want.

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