[Feature Request]: Support for System JS as output format

What problem does this feature solve?

SystemJs is used in quite a few microfrontend architectures. Typically these use Rollup with output format "system", as build tool. Are there any plans to support SystemJs output in Rolldown?

What does the proposed API look like?

import { defineConfig } from 'rolldown';

export default defineConfig({
  input: 'src/dummy.js',
  output: {
    dir: 'dist',
    entryFileNames: `[hash].system.js`,
    format: 'system',
  },
});

Results in: ERROR unimplemented: output.format: system