Add asyncStylesheets property to shell component

[i-like-robots]

In order to support the solution suggested in #546 we require a mechanism to load stylesheets in a non-blocking manner. This mechanism should be used to load stylesheets which are not required for the initial page render but supply styles for content which is less important, lazy loaded, or initialised by the user with JavaScript.

This feature was included with n-ui here but there may be other ways to achieve the desired behaviour.

References

• https://www.filamentgroup.com/lab/async-css.html
• https://www.filamentgroup.com/lab/load-css-simpler/
• https://discourse.wicg.io/t/a-standard-method-for-loading-style-sheets-asynchronously/542/4
• https://roots.io/guides/asynchronous-css-loading-in-sage/