Parallelize nested imports

[Rich-Harris]

On navigating to /foo, the router finds the component in question and discovers that it needs to do import('./foo-xyz123.js'). On loading that module, we may discover that the chunk depends on other chunks, which must then also be loaded.

Since we know the shape of the entire tree at build time, we could figure out all the chunks foo-xyz123.js depends on, and start importing them in the background.