Add img srcset parsing

Parse <code>img.srcset</code> per [parsing issue #7](https://github.com/microformats/microformats2-parsing/issues/7). MicroMicro now [supports](https://github.com/jgarber623/micromicro/releases/tag/v3.1.0) this, so once we have 2+ implementations and rough consensus, we can update the parsing specification.

(Originally published at: https://gregorlove.com/2022/09/add-img-srcset-parsing/)