Fix "nodenext" module resolution compatibility

[helgee]

What is the purpose of this pull request? (put an "X" next to an item)

• [ ] Documentation update
• [ ] Bug fix
• [ ] Feature
• [ ] Code style update (formatting)
• [ ] Refactoring (no functional changes)
• [ ] CI related changes
• [X] Other, please explain: Fixes a packaging/distribution issue

What changes did you make? (Give an overview)

Updated the type and exports fields of package.json to make the package compatible with "moduleResolution":"nodenext". Without these changes Typescript cannot resolve the default export with the above setting without workarounds. Encountered during: https://github.com/threlte/threlte/pull/1648

Is there anything you'd like reviewers to focus on?

No

Does this PR introduce a breaking change? (What changes might other developers need to make in their application due to this PR?)

No