Warm route code during idle time to speed rendering new pages

[KyleAMathews]

The first time a page component is used requires its code to be parsed & evaled. This happens after a link to the page is clicked at the moment which means the user waits for the 10-300ms required for this.

Ideally this parsing & evaling happens before the link is clicked so the user doesn't experience any delay.

This could be done by a similar mechanism to how we prefetch page resources. When we predict a page might be visited & all its resources are loaded — we watch for quiet CPU time and then simulate rendering the page which would cause webpack to load the code & run it. Then when the page is actually visited, it'll be rendered as fast as possible.