Add package.json to exports so it can be resolved

[wallw-teal]

Many tools attempt to require.resolve package.json, and in the latest versions of node that is not possible if the package specifies exports. The solution is to simply add package.json to exports.

Resolves #8992

[cesium-concierge]

Thanks for the pull request @wallw-bits!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[kring]

Thanks @wallw-bits!