Support the case where data-turbo-frame is specified in a link in the shadow DOM

[yuki24]

This PR adds support for an a element with the data-turbo-frame attribute within a shadow DOM.

@stedekay has provided a great explanation in #1021, but the TL; DR is that, the following example doesn't work without this patch:

import { html, LitElement } from "lit"

export class LinkElement extends LitElement {
  render() {
    return html`
      <a href="/path/to/link" data-turbo-frame="a-frame">
        <slot></slot>
      </a>
    `
  }
}

customElements.define('custom-link', LinkElement)

<custom-link>
  <!-- #shadow-root -->
  <!-- <a href="/path/to/link" data-turbo-frame="a-frame"></a> -->
</custom-link>
<turbo-frame id="a-frame"></turbo-frame>

closes #1021