programs/gammastep: add systemd service

[GetPsyched]

Adds gammastep.service

Note:

• I haven't added its sister service gammastep-indicator.service.
• I don't know much about systemd stuff so please nit if something's wrong or missing.
• This is the first time a systemd service is being added, I'm not sure what the guidelines are. Do we need tests? What kind of tests?
• My other PR, programs/winboat: init module #147, conflicts with this PR since it updates the lock file (or maybe not if the nixpkgs bump is distinguishable)

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AI used to generate code included in this PR?: No

All Submissions:

• Formatted commit message in accordance with CONTRIBUTING.md guidelines
• Filled in all meta items
• Mentioned any blockers before the PR can merge
• Verified there are no conflicting PRs open

New Module Submissions:

• Followed the general API laid out in CONTRIBUTING.md
• Wrote tests (or expressed a need for help on tests)

[eclairevoyant]

This yazi test is already failing on the current main branch, should be ignore-able wrt this PR.

NixOS tests are network-isolated, so the test itself is broken.
Fetching the schema files via FOD and using that would be better.