feat: add use theme

[josefaidt]

• adds theme action

  <script>
    import { theme } from 'svelte-themer/use'
  
    const myTheme = {
      text: 'red'
    }
  </script>
  
  <div use:theme={myTheme}>
    <p>Hello, World!</p>
  </div>
  
  <style>
    p {
      color: var(--text);
    }
  </style>

• adds createCSSVariableCollection
• renames createCSS to createCSSTemplate
• adds support to import support and use directly

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