feat: support customProps.cssUrls

[TatsuyaYamamoto]

Sorry for creating PR without asking in issues.

Motivation

I want my single-spa application be able to load css provided from root-config.

An application’s bundled css filename sometimes includes hash (index.abcdef.css). Hash text is detemined in build-time, so bundled js file cannot know hashed css file url. In case single-spa(-css), css urls received in opts.cssUrls cannot has build-time hash.

Example

// single-spa root-config
registerApplication({
  name: 'app1',
  activeWhen: '/app1',
  app: () => import('https://example.com/main.build-time-hash.js'),
  customProps: {
    cssUrls: [
      'https://example.com/main.build-time-hash.css', 👈
      'https://example.com/string-determined-after-build-time/main.css' 👈
    ] 
  }
});

// single-spa application
const cssLifecycles = singleSpaCss({
  cssUrls: [],
});

[filoxo]

Is this not already supported through the ExposeRuntimeCssAssetsPlugin as noted in the documentation here?

[TatsuyaYamamoto]

My understanding is that ExposeRuntimeCssAssetsPlugin is for webpack only.

In my case, I want to build single-spa application with Vite, so maybe I should create PR to provide vite-plugin. However, I think it would be better to support as a feature that is not dependent on a specific build tool.