Add `postBanner`/ `postFooter` option to inject content after minification

I still want to check esbuild's behavior in relation to this.

_Originally posted by @hyf0 in https://github.com/rolldown/rolldown/issues/6786#issuecomment-3472124695_

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Refs:

- https://github.com/vitejs/rolldown-vite/issues/459#issuecomment-3441031606