[worklets] Create ability for code to be loaded into worklets declaratively.

[bfgeek]

This is so that script doesn't have to be run to load code into an isolated worker. User agents can also do standard tricks like pre-fetching before html document is parsed.

This probably wants to be done with something like a tag. I.e.

<link href="paint.js" rel="paint-worker" />

[upsuper]

Probably something in CSS file like @import-worklet url;?

[bfgeek]

Service worker has done something similar with a link element. See:
w3c/ServiceWorker#828
http://mkruisselbrink.github.io/ServiceWorker/spec/service_worker/#link-type-serviceworker

For them they use a link element like:
<link rel="serviceworker" href="/js/sw.js" scope="/">

We'd probably do something similar like:
<link rel="workletname" href="/js/sw.js">

There was talk in the Sydney F2F that we might want to use the <script> tag instead, however it seems like, link is a better fit here.

@upsuper Yup, something in CSS would also be nice 👍

[atanassov]

From Houdini F2F Seattle - recommend adding a section that describes the feature in a declarative form even if it is not complete.

[domenic]

Let's consolidate this and the other related issues into whatwg/html#6073.