wpaperd: add systemd service; move to services/ from programs/

[3ulalia]

Description

Re: #4538, wpaperd needs something to run it after being enabled. This commit adds a systemd service to run it, and accordingly moves it to services.wpaperd.

In addition, the existing tests have been migrated to services, and an entry in the newslist has been created alerting users to this change.

@rycee pls i have deleted and recreated this PR 10 times to make it not conflict as the newsfile is updated ╥_╥

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC

@Avimitin