feat(astro): make an astro.js integration

[tremppu]

Atm Salty CSS already works with Astro as CSS-in-JS solution when first adding a React.js integration.

Steps to make Salty CSS in Astro happen before integration is live:

1. Add React integration to Astro https://docs.astro.build/en/guides/integrations-guide/react/
2. Initialize Salty CSS with CLI npx salty-css init
3. Install and add React and Vite packages manually npm i @salty-css/react @salty-css/vite
4. Add following vite configuration snippet to your astro.config:

// Import
import saltyPlugin from "@salty-css/vite";

// Config 
export default defineConfig({
  vite: {
    plugins: [
      saltyPlugin("dirname_or_path_to_your_repo"),
    ],
  },
});

5. Create new Salty CSS components with npx salty-css generate [dir]

Additionally className function works out of the box great atm with astro components.