Devcontainer: create simple setup using dev-server

Currently we still have the "precursor" to dev-server in `.devcontainer/`. This is complex code and difficult to understand and maintain. If we change the devcontainer setup to simply use dev-server, that would be much easier.

See here for an example:
https://github.com/UncleSamSwiss/ioBroker.i2c/blob/refactoring-2026/.devcontainer/devcontainer.json