New rule: Summary element has non-empty accessible name

[WilcoFiers]

New rule to test that summary elements have an accessible name.

I don't think this is covered by any other rule today, since summary elements do not have a semantic role.

Need for Call for Review: 2 weeks

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[giacomo-petri]

In Firefox, the ::marker CSS pseudo-element is part of the accessibility tree, which means the accessible name cannot be left empty in Firefox.

Should we address this, clarifying that the "triangle" icon is not enough?