Implement `querySelector` in Document

Although the API of this DOM implementation intends to be minimal, would you deem acceptable to implement only `querySelector` and `querySelectorAll` on the document to enable some usages in ember-fastboot?

Start by locating the Document entry point and checking how querySelector and querySelectorAll are represented in the DOM implementation. Confirm the minimal API needed by ember-fastboot, then verify that both methods work on the document and that the relevant existing tests pass or add coverage for the requested behavior.

Written by the indexing model from the issue text.