Include roles from shadowDOM in Error message

[kilmc]

What:
Adds a check which improves Error messages for folks testing Web Components using getByRole.

Note: This PR is focused on getting a correct Error message and not a passing test. This fix will be most useful to folks using dom-testing-library in conjunction with the wrapper library referenced in this comment in #413.

Why:
When testing Web Components the returned error is incorrect. The example below walks you through this case.

<div> <h2>An example</h2> <custom-button /> </div>

Contents of the shadowDOM for <custom-button>

<button>Button text</button> 

Below is a test assertion with a minor typo in it.

expect(screen.getByRole('button', { name: 'Button texx' })).toBeVisible() // Throws an error

Before the change in this PR the error message returned is:

There are no accessible roles. But there might be some inaccessible roles. If you wish to access them, then set the `hidden` option to `true`. Learn more about this here: https://testing-library.com/docs/dom-testing-library/api-queries#byrole 

After the change the error becomes:

"Unable to find an accessible element with the role "button" and name "Button texx" Here are the accessible roles: button: Name "Button text": <button /> -------------------------------------------------- 

How:
Adds a check for node.shadowRoot in flattenDom to ensure children inside of Web Components are traversed.

Checklist:

• Documentation added to the
  docs site N/A
• Tests
• TypeScript definitions updated N/A
• Ready to be merged

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[kilmc]

Is there anything I can do to move this along, even if it's going to be rejected?