cliphist: support multiple systemdTargets properly

[khaneliman]

Using a space separated list of targets as a single string element in the list doesn't work properly. Change property to support list of targets and backwards compatibility with warning for single string.

Description

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

[khaneliman]

@rycee any chance you could take a look at this ?

[stale]

Thank you for your contribution! I marked this pull request as stale due to inactivity. Please read the relevant sections below before commenting.

If you are the original author of the PR

• GitHub sometimes doesn't notify people who commented / reviewed a PR previously when you (force) push commits. If you have addressed the reviews you can officially ask for a review from those who commented to you or anyone else.
• If it is unfinished but you plan to finish it, please mark it as a draft.
• If you don't expect to work on it any time soon, please consider closing it with a short comment encouraging someone else to pick up your work.
• To get things rolling again, rebase the PR against the target branch and address valid comments.

If you are not the original author of the PR

• If you want to pick up the work on this PR, please create a new PR and indicate that it supercedes and closes this PR.