Figure out what rules need to support shadow DOM

[WilcoFiers]

We need to go through all of the rules and identify which ones need to change for them to work from within a shadow tree. For each of these, a ticket must be created.

[WilcoFiers]

I've put together tickets for rules that need to be updated for

"aria-required-children" check to use the composed tree #421
"aria-required-parent" check to use the composed tree in getAriaOwners #422
"caption" and "description" checks to search for <track> in composed tree #423
"landmark" check should look into shadow elements for <main> #424
"fieldset" check to use the composed tree #425
"group-labelledby" check to use the composed tree #426
"only-dlitems", "structured-dlitems" and "only-listitems" checks must look at composed children #427
"dlitem" and "listitem" checks to use getComposedParent #428
"duplicate-id" to allow repeating IDs in different contexts #429
"duplicate-img-label" check to consider texts/images on composed tree #430
"explicit-label" check to look in the correct context #431
"link-in-text-block" check to look up the composed tree #432
"region" check to look at the composed tree #433