Provide an ES Module version

[SaekiRaku]

I want to deal with JSX in Deno, but it can only import ES Modules.

For now, I just download the acorn-jsx and change all exports/require syntax to export/import syntax, it works for me. But it's better to import from a CDN link directly in Deno.

[RReverser]

I think ES module version would be useful (regardless of Deno) and doable in a similar way to what we do in Acorn; I'll be happy to accept such a PR.