add "core" package to API package as a peerdep

[josefaidt]

Description of changes

The core package is consumed by api but is not declared as a dependency in the api's package.json

https://github.com/aws-amplify/amplify-js/blob/main/packages/api/src/API.ts#L5
https://github.com/aws-amplify/amplify-js/blob/main/packages/api/package.json#L70-L74

This change modifies the dependencies of the API package to mimic its dependencies https://github.com/aws-amplify/amplify-js/blob/main/packages/api-rest/package.json#L86-L90

• mimics dependency setup of @aws-amplify/api's dependencies: api-rest and api-graphql
• adds @aws-amplify/core as a devDependency to @aws-amplify/api
• adds @aws-amplify/core as a peerDependency to @aws-amplify/api

Issue #, if available

Description of how you validated changes

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.