[Feature] Add export information in the metafile

[remorses]

I need to have access to export information, this could be done in the metafile output, something like this

{
  "outputs": {
    "react/index.js": {
      "exports": ["default", "namedExport", "anotherNamedExport"],
      "imports": [
        ...
      ],
      "inputs": {
        ...
      },
      "bytes": 119
    }
}

My use case is detecting if a bundle is a commonjs module, i can know this checking if a module has only a default export

[evanw]

This sounds good. I will get this in sometime.

[remorses]

Thank you for your amazing work ❤️