Incorporate ./shells

[blaggacao]

Is your feature request related to a problem? Please describe.
I think it is time to share efforts in building user provided dev environments. I also think this template repo leverages the flakes model of sharing to the most elaborate extend. I would be pleased if I could use something like github.com:user/nixflk#devShells.rust of a trusted user as an input to my own variety of devshells.

Describe the solution you'd like
I would like such sharing to evolve around https://github.com/numtide/devshell to provide an advanced and consistent devshell environment.

Describe alternatives you've considered
none — inmature thought.

Additional context

• https://github.com/numtide/devshell

• https://github.com/colemickens/nixcfg/tree/main/shells (for what I mean)

• User provided dev environments would be oposed to porject provided dev environments. It would be interesting to learn about ways how user provided dev environments can hook into / overlay / complement project provided ones. This question collaterally seeks answer here: Document how to share devshell modules numtide/devshell#52

• a few shell.nixs for being able to hack on random source code trees

  — for a quite intriguing motivation to boost collaboration on source code through the flakes model

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[nrdxp]

I like the idea of providing various shells as an output. I've had a similar idea recently. I think the simplest way to go about this would be to add a custom flake output attribute. Something like shells. We will probably have to write some checks as well.

[blaggacao]

I hope @zimbatm could add to the thought-noodle-soop we are cooking here. Should we add more salt? A shells attribute is already quite spicey. But will it be allowed once / if flakes outputes will be typed?

[nrdxp]

#54 is a step in this direction.

[blaggacao]

I try to second: #55