Statically generate index.html files for each route when using history API

[agubler]

Enhancement

Currently build time rendering supports applications that use hash routing. This runs the application bundles and writes the html and critical css for each route to the applications index.html. For hash routing we support multiple routes by injecting a small script into the index.html that appends the correct html segment depending on the route.

However build time rendering does not currently support applications using the history API for routing (StateHistory provided by @dojo/framework/routing). Build time rendering needs to be enhanced such that we generate a separate index.html in the directory that reflects the route in the build output:

1. The static HTML for the route is written to the index.html.
2. Critical CSS required to load the route is added to a style in the index.html.
3. Relative src references in the HTML are amended to ensure that resources are loaded correctly.
4. Relative url references in the CSS are amended to ensure that resources are loaded correctly.
5. The src of the scripts/css are amended with a relative prefix so they can be loaded.