Add knip to CI checks

[with-heart]

can this be added as a CI check?

_Originally posted by @Andarist in https://github.com/statelyai/xstate/pull/4974#discussion_r1677358370_

In order for this to happen, we first need to:

• [x] resolve "Referenced optional peerDependencies" warnings (not sure why these are reported)
• [x] resolve "Unlisted binaries" warnings (svelte-check and docs:build references in actions/workflows)
  • svelte-check is a dep of xstate-svelte so I think that might be why it errors
• [ ] add action for knip to ci-checks/action.yml