Pass `esm: true` flag to NodeJS package

[andresgutgon]

What?

Hi, I'm building my SSR JS code as ESM modules. With the current version of this package, I manage to do this by naming the generated file as ssr.mjs, which is nice.

But I notices elixir-nodejs package added the option to serve ESM modules here

I think it would be just to let users from this package specify that option.

I can do the PRs myself if that helps. Or maybe what I'm saying is not correct, so happy to be corrected.

[andresgutgon]

Ok, I think the change was pretty easy so I made the PR : )