Clear and concise description of the problem

Description

It would be useful to have banner and footer configuration options similar to tsup (esbuild) in tsdown to allow inserting custom content (like comments or notices) at the beginning and end of output files.

Use Cases

1. Adding warning notices about generated files
2. Including license information
3. Adding build metadata or timestamps
4. Inserting environment-specific configuration

Related

• Similar feature in tsup: Support banner and footer esbuild options? egoist/tsup#451

Suggested solution

Proposed API

Similar to tsup / esbuild:

export default defineConfig({
  // ... other config
  banner: {
    js: `/**
     * ⚠️  WARNING: This is a generated file
     * Do not edit this file directly.
     * Generated: ${new Date().toISOString()}
     */`,
  },
  footer: {
    js: `// End of generated file`,
  },
})

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guide.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.