Add a file to the `yoastseo` package that specifies which morphology file version it works with

[atimmer]

Explanation

For fixing the request of the morphology file yoastseo needs to keep track of which morphology file version it works with. We do this with semver contraints. In our case the use of the ^ is the best. So the file could look something like this:

{
    "morphology-en": "^1.0.0",
    "morphology-de": "^2.0.0",
    "morphology-nl": "^1.0.0"
}

Technical decisions

The data in this file should be exported as a constant in the public API, because the plugin needs to read it and send it to MyYoast. Also the TYPO3 module also needs to be able to use this.

Feedback?