Implement tree assessor factory functions

[hansjovis]

Summary

This PR can be summarized in the following changelog entry:

• [non-user-facing] Implements a collection of factory functions that create TreeAssessors for different kinds of SEO and readability analysis. The kind produced depends on factors like type of content (normal page/post, cornerstone, taxonomy page) and type of keyphrase (main or related).

Relevant technical choices:

I made some JSDoc submodules to keep relevant content together, whilst keeping them from all getting on the tree/assess module. I am not quite sure about the naming, so a second opinion would be nice!

• The factory functions live in their own JSDoc module called tree/assess/assessors.
  • Cornerstone functions live in tree/assess/assessors/cornerstone.
• I separated creating the assessment lists into separate factory functions, since they can grow quite big. They live in the submodule tree/assess/assessors/assessments and tree/assess/assessors/cornerstone/assessments respectively.

Test instructions

This PR can be tested by following these steps:

• Run yarn test, check if the coverage is 100% for the tree/assess folder.
• Check if the tests cover all edge cases.
• Review the code a second time, check if you can find any missing functionality or code improvements.

Fixes #2126

[nataliashitova]

This PR has been transferred to the new monorepo https://github.com/Yoast/javascript/pull/6.