[qunit] Add ES6 import support

[mike-north]

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This PR makes it possible to explicitly import from QUnit using ES6 module syntax

import { module, test } from 'qunit';

This involved some significant refactoring because the previous state of these types involved no exports (adding the first export broke the implicit availability of a global QUnit namespace)

Additionally, in response to user feedback, I've added some API documentation to the Hooks and NestedHooks interfaces, in an effort to avoid possible confusion between the two.

cc: @scalvert

[typescript-bot]

@mike-north Thank you for submitting this PR!

🔔 @waratuman @sechel - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[typescript-bot]

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[scalvert]

Thanks for this, @mike-north!