Export SeoAssessor from project index

[Kingdutch]

Summary

This PR can be summarized in the following changelog entry:

• The SeoAssessor is now exported from the project again.

Relevant technical choices:

• This was exported previously I believe but removed in the refactoring to import/export.

Test instructions

This PR can be tested by following these steps:

• Install yoastseo through NPM
• Try to import { SeoAssessor } from 'yoastseo';
• See that webpack happily compiles

Fixes #1878

[hansjovis]

Acceptance test: ✅

Thanks @Kingdutch for the patch! People like you power the awesomeness that is open source.