Ignore a11y checks on child components

[taylorzane]

I just encountered an interesting "bug" of the a11y implementation. I have a component that uses scope as the prop name, and I get a warning that The scope attribute should only be used with elements. I feel that a11y should be disabled on component definitions. Here's the full error (including code)

src/Components/Foo/index.html (20:71) A11y: The scope attribute should only be used with <th> elements
18:   <div id="foo-id">
19:     <div class="foo-class">
20:       <Child scope="local"></Child>
                 ^
21:     </div>
22:   </div>

[Conduitry]

At least until Svelte v2, we are merely suggesting and not enforcing that component names begin with a capital letter and regular elements begin with a lowercase letter. So we have to check whether the given element is actually intended as a component before skipping the a11y checks. validateElement.ts has a check for this near the top (node.name === ':Self' || node.name === ':Component' || validator.components.has(node.name)) which perhaps should be copied into a11y.ts, or perhaps which should be extracted into some utility function, or perhaps we should refactor that check out into validateHtml in index.ts somehow.

[Rich-Harris]

As of 1.52 component attributes are skipped during a11y checks — thanks