Add posthog-core to Rollup config

[harrison-carter]

Is your feature request related to a problem? No

Describe the solution you'd like

To use posthog-js-lite as a dependency within our project, it would be helpful to be able to import posthog-core directly from this NPM package.

Importing the posthog-core package to be used in a Shopify WebPixel is the use case
https://posthog.com/docs/libraries/shopify

Describe alternatives you've considered

I am currently copying the code directly from the repo which isn't useful for dependency / version release management in order to use the PostHogCore

Related sub-libraries

• All of them
• posthog-web
• posthog-node
• posthog-react-native

[harrison-carter]

Raised as a PR
#340