Allow to use scoped (':scope' and '&') selectors relative to controller's element in outlets selectors

[christophehenry]

This allow to use the concept of outlets to express component composition. Take the following:

<CustomSelect id="products" data-controller="custom-select" data-custom-select-custom-item-outlet="d .items">
    <SearchBar role="searchbox"></SearchBar>
    <ol>
       <li>
           <CustomComponent data-controller="custom-item" class="items"></CustomComponent>
           <!-- Some more-->
       </li> 
    </ol>
</CustomSelect>

This allows to reference custom-item outlets only within the scope of custom-select controller's element. Of course you could solve this by setting an id on every custom-select controller's element and referencing it in outlet's selector but this is not always convenient since HTML code can be generated and not easy to customize. Plus, scope selectors (& and :scope) are a things in most browsers now.

What it does is, if element.matches(selector) returns false, the code will additionnaly perform a querySelectorAll(selector) on the controller's element nand tests if the element appears in the results. This is additionnal work but I expect performance impact to be very limited since this additionnal test will only be performed if the first match fails, I expect querySelectorAll to be highly optimized on modern browsers and I expect the controller's element to have a limited nuber of nodes mode of the time. The performance penalty could, however, be mitigated by pushing whatwg/dom#1081 into the standards.