Add direct CSS loading and static file loading to module server

[calebeby]

Before this PR, CSS was supported in this way:

You could use loadCSS:

await utils.loadCSS('./something.sass') // or css, etc

You could use css imports from JS files:

import './asdf.css'

The CSS file would be piped through PostCSS and converted into a JS script that injects the CSS as a <style> tag.

Also, sass/less/stylus/less were supported (via rollup-plugin-postcss):

import './asdf.sass'

However, if you requested a sass/css file via HTTP, you would get back a JS file with the code to inject the CSS. This is not good.

Now, in addition to those use cases, CSS is supported at the HTTP server layer, so if you request a CSS file via something other than a JS import, it will end up being a CSS file rather than a JS file.

This is done by adding ?import to imports in JS files to distinguish between JS/CSS (same as Vite does, and similar to what WMR does):

import './asdf.css' // => transpiles to import './asdf.css?import'

Then the HTTP server knows to return a JS file with the CSS injector if ?import exists, and CSS directly otherwise.

An example use-case for this is if @import is used

Also, the HTTP server now supports static assets (like fonts/images loaded via CSS - which I ran into on cloudfour.com)

[gerardo-rodriguez]

Nice! 🎉