Add devenv.sh based development environment

[ngiger]

About three years ago I switched from Debian to NixOS and discovered a bit later devenv.sh.
Each time I work on project I try to add a devenv environment to avoid any issues with conflicting libraries, compilers, databases, applications, etc. Devenv allows me to get a reproducible environment (similar as running a docker image). It is handy to define needed background process like a mysql or postgres database with correct username/password combinations. All in a even more compact form than a docker image.

Nobody should feel forced to use it, but it would be nice for me, if it could go into master and last stable branches. This would make it a lot easier for other people to try it.

[col-panic]

To realize this, we use both

• Eclipse Installer with its setup file Elexis.setup to have a shared and equal installation of the Eclipse development environment and
• Docker to share the build environment, please see the discussion https://github.com/orgs/elexis/discussions/1 for this.

We recently devoted some time to clean up all the artefacts we didn't know about in order to be on the same page.
Our goal was that for all artefacts in the repo, we have at least 2 persons who use it and know how it works.

Please keep it in a fork of the repo for the time being. You could teach us on one of the next Elexis Dev Meetings and
if you convince us and we have at least 2 persons, then we'll adopt it to core!

@debenol1 what do you think?

[debenol1]

Although devenv is an interesting, container-free tool for providing standardized development environments, I would keep the technology stack for Elexis as slim as possible and focus on Eclipse Installer/Docker. I support the idea of ​​a separate fork for devenv