[w3c/manifest] Support different shortcut icon requirements/guidelines on different platforms (#795)

Spun off from #768.

@christianliebel :
> The platform conventions for shortcut icons seem to differ quite a bit. For instance, iOS shortcuts use icon masks; Windows jump list entries have colored icons, etc.
>
> Is there a plan on how to deal with this? Should a user agent on IOS create an icon mask from a given image? And/or should `ImageResource`’s `platform` member be evaluated for that scenario?
>
> I assume this question will arise very quickly, so it would be great if the spec could be a bit clearer on this.

Me:
> Yeah, since shortcut uses `ImageResource`, maskable should cover any platform-specific needs with regards to conforming all icons to a specific shape. We don't currently have a solution for platforms that want icons to be all monochrome. It's possible there's a more pressing need for that in relation to shortcuts than generic app icons (since some platforms may have monochrome shortcut icons).
>
> If we want to go down that path, I would propose a new `purpose: "monochrome"` for ImageResource (that would be separate to this PR). I want to avoid any ability for developers to supply an "iOS icon" and a "Windows icon", etc, because then we will break away from the Web platform's platform neutrality and start seeing apps with poor or no icons on other platforms. (Devs can already technically do this by user-agent-detecting when they serve their manifest, but I want to avoid giving them an explicit feature to do this.)

I suggested that we start with an analysis of what shortcut icon guidelines there are on each major platform so we can see whether "monochrome" is necessary and sufficient (which should be done here perhaps).

@beverloo :
> Would the `badge` purpose be sufficient? It specifically leaves such flexibility as we have the same constraints in other UI surfaces:
>
> > A user agent can present this icon where space constraints and/or color requirements differ from those of the application icon.

Let's continue the discussion here.

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