feat: Expose react option for params

[joelostblom]

PR Description

Currently the Vega-Lite schema does not officially support the use of the option react to params. It still works to use it in the online editor, but it raises an error when validating the spec in Altair. We want to use it to create a constant color domain like in this example originally from vega/altair#3394 (comment). Adding this functionality would allow us to have a constant color domain even when filtering which is very useful (and could even be a good default option for using the legend to filter colors). Without setting a constant domain, it is not possible to click additional categories since they disappear from the legend as described in #9360

close #9360

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.

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[joelostblom]

This is now ready for review and I updated the description in the original comment with more details.

[joelostblom]

Thanks for the review @domoritz , just checking if I can merge this or if you left it open for someone else to have a look first?

[domoritz]

This pull request just passes the signal property through: https://vega.github.io/vega/docs/signals/. Looks good.