Allow npm build to manipulate shipped filesystem (support Critical CSS, etc)

[timkelty]

Currently, the NPM build exclusively ships artifacts to the CDN.
This means if the build process writes things outside of the webroot, it will be ignored, as the resulting files are not included back into the shipped filesystem.

Use-cases include:

• Critical CSS html templates
• Fractal (or other component library) builds (twig templates)
• Front-end build tools that write Twig templates, eg https://github.com/brianjhanson/vite-plugin-craftcms

/cc @shennyg