feat: support copy builder

[dwightjack]

🔗 Linked issue

• Resolves: add copy builder #266

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR adds a copy builder, very similar to the mkdist one. Similarly to mkdist, the builder accepts input, outDir and pattern options. Example:

import { defineBuildConfig } from "unbuild";

export default defineBuildConfig({
  entries: [
    {
      builder: "copy",
      input: 'src/assets',
      pattern: '*.png', // defaults to '**'
      outDir: 'dist'
    },
});

It also provides the following hooks:

• copy:entries: to hook to the entries array
• copy:done: to hook to the context once the copy process has completed.

If you feel this initial implementation could be useful, I can update the documentation with an example.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[pi0]

Thanks!