[w3c/manifest] Processing steps change the types of things (type errors) (#817)

Spun off from w3c/image-resource#8.

It seems there are "type errors" (in a spec sense, not like JavaScript `TypeError` problems) all over the manifest spec, since we process various members, changing their type, and store them back in the manifest WebIDL structure.

For example, `start_url`:

* It's specified as `USVString start_url` in the [`WebAppManifest` WebIDL dictionary](https://www.w3.org/TR/appmanifest/#webappmanifest-dictionary).
* The [steps for processing the `start_url` member](https://www.w3.org/TR/appmanifest/#dfn-processing-the-start_url-member) convert the USVString into a URL object, returning the URL.
* The [steps for processing a manifest](https://www.w3.org/TR/appmanifest/#dfn-processing-a-manifest) say "Set manifest["`start_url`"] to the result of running [processing the `start_url` member](https://www.w3.org/TR/appmanifest/#dfn-processing-the-start_url-member)", technically storing a URL object in a field of type USVString.

This sort of type error happens everywhere where a processing step changes the type of a member. They noticed it in the Image Resource spec, and the solution there was to define a "processed image resource" structure, capable of holding the transformed types. If Manifest were to switch over to using the Image Resource spec, we would now be forced to shove a "sequence of processed image resource" in several fields of type `sequence<ImageResource>`, but that wouldn't be any worse than the existing type errors.

Ideally, we would similarly define a "processed web app manifest", but that would be quite a lot of work and redundancy in the spec, which I'm not sure we want to maintain. A quick hand-wavey solution may be to update the existing term "[processed manifest](https://www.w3.org/TR/appmanifest/#dfn-processed-manifest)" to say that it is a structure with the same members as `WebAppManifest`, but whose types may be different, for example `start_url` is a URL, not a USVString.

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