Refactor core code into a separate package

[hsubox76]

Create a separate "core" NPM package for a future SDK to consume, to share code with this one.

Since it lives in the same repo as the google/generative-ai package, the generative-ai SDK will consume it through local paths instead of as a package dependency, and be able to take advantage of a shared folder for tests, etc. Other SDKs will consume it as an NPM installed package.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 44f4f58

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR