nixos/startx: try to improve UX

There are some common pitfalls and no documentation around how to write
the .xinitrc to correctly start the window manager, the systemd
graphical session and, ideally, cleaning up afterwards.

To improve the user experience around startx this change:

1. Adds two options to generate a sane default script and extend
   it declaratively from NixOS.

2. Adds assertions to graphical-session.target so that it will fail
   clearly and immediately when users writing their own script forget to
   import the necessary environment variables.

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