Re: [fetch] Introduce convenience property for request's context (#93)

`script` and `worker` also differ in terms of their capability, as the former has access to the DOM, and the latter has it's own execution context and therefore its own Content Security Policy. I'd suggest that distinguishing between them is reasonable.

For CSP and MIX, I don't think we particularly care how the request was _initiated_. We care more about how the content will be _used_, and we're using things like "The request came from an `<img>` tag" as a proxy for that detail.

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