[w3c/manifest] Icon matching algorithm: unrecognised purposes can result in "any" (#728)

https://w3c.github.io/manifest/#purpose-member

Currently, if the user agent doesn't recognise any of the purposes in the list, it defaults to `"any"`. Instead, this should be an error (i.e., the icon should be ignored). The default of `"any"` should only be applied if purpose is the empty string or missing. Otherwise, it is still permitted to have unrecognised purposes in the list, but the UA must only treat the icon as one of the recognised purposes.

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