feat: add function type for options.injectStyle

[await-ovo]

Support setting options.injectStyle as a function that returns JS code to add CSS in a custom way, like rollup-plugin-postcss.

This can be used in some cases such as when adding styles to shadowRoot:

export defineConfig({
   injectStyle: (css) => {
    return `
        var setStyle = function() {
          setTimeout(() => {
            try {
              var style = document.createElement('style');
              style.type = 'text/css';
              style.innerHTML = ${css};
              var root = document.getElementsByTagName('${tag}')[0].shadowRoot;
              root.insertBefore(style);
            } catch {
              setStyle()
            }
          },1000)
        };
        setStyle();
      `;
  },
})

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Name	Status	Preview	Comments	Updated
tsup	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Mar 10, 2023 at 10:05AM (UTC)

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🎉 This PR is included in version 6.7.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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