LibWeb: Assign “orphaned” li elements the default ARIA role “none”

[sideshowbarker]

This change makes Ladybird conform to the current requirements at https://w3c.github.io/core-aam/#roleMappingComputedRole in the “Core Accessibility API Mappings” spec for the case of “orphaned” li elements; that is, any li element which doesn’t have role=list ancestor.

The core-aam spec requires that in such cases, the li element must not be assigned the listitem role but instead must be treated as if it had no role at all.

This change gets us 100% passing the https://wpt.fyi/results/html-aam/roles-generic.tentative.html?product=ladybird test.