Add support for <summary> elements

[KittyGiraudel]

This pull-request adds two additional selectors to consider <summary> elements.

1. This considers the first summary element directly within a details element focusable, provided it doesn’t have a negative tabindex attribute.

details > summary:first-of-type:not([tabindex^="-"])

2. This consider the details element focusable provided it does not have a summary element and does not have a negative tabindex attribute.

details:not(:has(> summary)):not([tabindex^="-"])

Find more information about this in the HTML specifications.

[KittyGiraudel]

@mxmason for review. :)