Related to rolldown/rolldown#2387, I'm experimenting with running systemjs transform inside legacy plugin as it already has babel plugin transform.

I found a few things rolldown is missing as rollup compat. Not working yet, but it looks doable by working around on plugin side.

rollup compat issues

• missing "system" format
  • we can do "esm -> system" transform per chunk (e.g. by babel)
• multi output format build
  • rolldown does two separate builds, but the plugin still works? [Bug]: output array results in multiple build runs rolldown/rolldown#3274
• bundle[filename] = legacyPolyfillChunk to add an entire chunk is not supported
  • emitFile({ type: "asset" }) is enough?
• how to track outputToChunkFileNameToPolyfills, which relies on object identity and mutation of js plugin arguments?
  • can use a simpler way to differentiate modern/legacy build?
• option object mutation opts.__vite_skip_esbuild__, __vite_force_terser__, etc...
• missing meta.chunks in renderChunk(..., meta) [Tracking] Rollup Plugin Compat Status rolldown/rolldown#819
  • this can be also simplified without it?