swap out devenv for vanilla flakes with templates and custom utilities

[callumcurtis]

devenv is useful for reducing boilerplate but has some drawbacks, such as poor support for .env files [1, 2].

It seems much more sustainable to use vanilla flakes but maintain a library of templates and utilities to reduce the overhead in creating new flakes for projects.

Criteria:

• [x] Replace use of devenv in firehose
• [x] Remove devenv from user features in this repo

Create templates and utilities as needed.

[callumcurtis]

I'll stick with devenv in firehose for now to learn where it helps/hurts.

[callumcurtis]

Removed devenv from user features and firehose.