[worklets] Provide hook for allowing downstream specs to specify a CSP destination.

[bfgeek]

For example the css-paint-api spec should provide its worklet with a destination of "paintworklet".

Being more granular probably makes more sense here, instead of just a generic "worklet" destination.

[bfgeek]

Sent whatwg/fetch#527 for fetch integration.

[bfgeek]

And sent w3c/webappsec-csp#205 for CSP integration.

[padenot]

What is preventing this to be merged, @bfgeek ? The two PRs above are merged.