feat: add support for signals in mark clip

[lsh]

PR Description

In Vega, mark supports a signal for the clip property. To round out the Vega-Lite API, we should support it here as well, especially since it is trivial to add.

Checklist

• This PR is atomic (i.e., it fixes one issue at a time).
• The title is a concise semantic commit message (e.g. "fix: correctly handle undefined properties").
• yarn test runs successfully
• For new features:
  • Has unit tests.
  • Has documentation under site/docs/ + examples.

Tips:

• https://medium.com/@greenberg/writing-pull-requests-your-coworkers-might-enjoy-reading-9d0307e93da3 is a nice article about writing a nice PR.
• Use draft PR for work in progress PRs / when you want early feedback (https://github.blog/2019-02-14-introducing-draft-pull-requests/).

[kanitw]

Thanks. Have a nit to fix before merging.