Add export map support

[nolanlawson]

This is a new thing in Node.js: https://nodejs.org/api/esm.html#esm_package_entry_points

We could use this to formalize the .cjs versus .js versions in some of the APIs.

I don't know which versions of Node actually support this, though.

[nolanlawson]

This may technically be a breaking change, as it restricts access to files that used to be exposed.

[nolanlawson]

Not sure this is super necessary, since I already only publish JS files that are actually exposed as part of the API.

[nolanlawson]

The only argument I can think for doing this is to make the relationship between trimEmojiData.js and trimEmojiData.cjs explicit. Every file exposed was meant to be exposed: https://unpkg.com/browse/emoji-picker-element/