fix: add typings for alternative entrypoints

[levrik]

These were present in @types/mixpanel-browser before but got lost when inlining them. I originally introduced them in the external types through DefinitelyTyped/DefinitelyTyped#72165.

Fixes #501

[jakewski]

Hey thanks for the contribution!

If I'm understanding correctly, the use case here is to be able to import the pre-built bundles like import mixpanel from 'mixpanel-browser/dist/mixpanel-core' and the types are not applied? Also, does this PR build the files in dist or were they manually added? Everything in dist/ is generated code so just making sure npm run build-dist doesn't override all of your changes here.

[federicobond]

If I'm understanding correctly, the use case here is to be able to import the pre-built bundles like import mixpanel from 'mixpanel-browser/dist/mixpanel-core' and the types are not applied?

This is correct. It's currently preventing me (and likely others) from updating to the latest version of the mixpanel library.