Add a bundle size reporter

[aleclarson]

This package may come in handy here: https://www.npmjs.com/package/gzip-size

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[OmgImAlexis]

Duplicate of https://github.com/egoist/tsup/issues/443?

[egoist]

@OmgImAlexis it's this part, I haven't closed this issue because it doesn't show gzip size yet and I'm not sure if it's necessary