Wil submit a PR for this.
Curious how you plan to do this :)
But I'm also not sure why you'd need to know. What would you want to do different if the browser supported CSP or not? You can also just send the header regardless if the browser supports it or not.
See the PR for the test. One major use case is for MVC frameworks. Many use eval() or new Function(string).
Developers need to be able to check if those will be able to work properly then adjust accordingly.
I suppose we should close this as the feature detection has moved into the w3c security wg