Evaluate additional Yoast researchers (functions)

[Sebobo]

The Yoast library contains many additional functions which are not automatically used by the default app or assessors used. But some of them could be actually quite helpful.
There it makes sense to check them out and decide whether to integrate the function or do it later or never.

Researchers:

• readingTime -> helpful information for frontend and backend
• relevantWords -> maybe helpful to advise what keywords could be used. See Show relevant words which Yoast Plugin can provide #17

Readability:

• getTooLongParagraphs -> maybe could be used to highlight problematic paragraphs
• tooLongSentences -> similar to problematic paragraphs

This list is not finished yet and might be extended in the future. Changes should be connected to their PRs.