handlebars.cli: add --esm 'handlebars' option for ES Modules support …

[Laoujin]

…(import handlebars from 'handlebars') for use in modern app/typescript

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[Laoujin]

I didn't get any of these (#1528) to work for me (using TypeScript and "type": "module")
Instead of updating the output manually, I created this PR 😀

If you'd like, I can also update /docs

[Laoujin]

I just went ahead and added it to the docs:
https://github.com/handlebars-lang/docs/pull/165/files

If you want it differently, just let me know :)