Add nix support

[acaloiaro]

This PR allows air to be a Flake input for Nix users who use air in their Go projects.

It also better supports air development by providing a development shell that contains all of air's developer dependencies.

I understand if this is not a priority for this project, and I'll continue maintaining my fork of air at acaloiaro/air if you choose not to directly support nix.

Cheers!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (56d3d58) 69.22% compared to head (57f33b5) 69.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #519      +/-   ##
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+ Coverage   69.22%   69.50%   +0.27%     
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  Files           9        9              
  Lines        1082     1082              
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+ Hits          749      752       +3     
+ Misses        253      251       -2     
+ Partials       80       79       -1     

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[cosmtrek]

@acaloiaro Thanks! Looks great, while I need some time to learn Nix firstly 😄

[acaloiaro]

@acaloiaro Thanks! Looks great, while I need some time to learn Nix firstly 😄

Sounds good @cosmtrek. I'm happy to clarify anything if you have questions.

One thing worth considering for this PR is that it serves two uses cases: users and developers.

For users it makes air available as a "flake input" for nix projects that use air. E.g. this allows Nix Flake users to delcare air as an input to their system configurations (make it globablly available), or as an input to projects, so that air is available within that project's "development shell".

For air developers who are also nix users, it allows them to simply run nix develop to get a development shell with all of air's dependencies.

I joined these two user and developer use cases into a single PR, but they could be easy considered in isolation. Selfishly, as a nix user, I want both. But it would be reasonable as a project maintainer to consider them separately.