This PR adds a devenv template based on [devenv.sh](https://devenv.sh/),
this is a nice to have for devs that use nix, direnv, and devenv.
It provides a quick and easy way to ensure that all packages that are
needed to start up the dev environment are installed.
I've initialized using devenvious, which is a @tennox creation: [source
on gitlab](https://gitlab.com/txlab/dx/devenvious).
I acknowledge that this "clutters" the root dir with some less common
files, but they are harmless for those that don't use the tools, and
helpful for those that do... a bit like .vscode and .idea folders.
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Co-authored-by: Félix Malfait <felix@twenty.com>
