feat: add esm entry point

[seanparmelee]

Summary

I noticed the build produces an amplitude.esm.js file, but is not referenced in the package.json. This PR adds the module entry point to the package.json so folks using ESM along with bundlers like webpack, parcel, etc will automatically use the ESM version of amplitude rather than the UMD/CJS version.

Checklist

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• Does your PR have a breaking change?: No

[taytestokes]

It appears that trying to use a named import of the Identify constructor from amplitude-js breaks after this change.

import { Identify } from 'amplitude-js'

The project that I was working with was currently on v8.8 and the named import of the Identify constructor was working, but after updating to v8.11+ it failed with a type error that Identify wasn't a function.

The resolution was to target the Identify constructor through the prototype of the defaulted export which gets set here:

Amplitude-JavaScript/src/amplitude.js

Line 28 in 064944c

Amplitude.prototype.Identify = Identify;

Example of use through the prototype:

addUserProperty(property, value) {
    const identify = new amplitude.Identify().add(property, value);
    this._instance?.identify(identify);
  }

Maybe I am missing something about how the imports/exports should work after setting the module field in the package.json, but I just wanted to bring this up.

CC: @seanparmelee @qingzhuozhen

[seanparmelee]

Sorry about that @taytestokes. I'll defer to one of the Amplitude devs on whether it's expected that Identify isn't exported as a named export, but looking at the docs, it seems like the expectation is to use new amplitude.Identify()

https://developers.amplitude.com/docs/javascript#setting-a-user-property
https://amplitude.github.io/Amplitude-JavaScript/Identify

[taytestokes]

No worries @seanparmelee! Thanks for the quick reply. :) From looking at the code briefly, It doesn't seem like it should be a named export so maybe I missed something there. As well as looking through the docs earlier it did seem like the expectation was to use new amplitude.Identify() so I wasn't entirely sure where the named import pattern came from and how it was working, but I just thought I'd share my findings!