feat(ListItem): Add item and position properties
feat(ListItem): Add item and position properties

Also make a `Thing` so that it has a `url` property. This allows us to generate HTML Microdata for list items that conforms to https://developers.google.com/search/docs/data-types/carousel#datatypes

See https://github.com/stencila/encoda/blob/9190db9fbc77510c73359b4a53fca9b1977e23a0/src/codecs/html/index.ts#L1606

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