Make Document Element Types final

[scrummer]

Task

The goal is to clean up the baseline and get rid of all the ignored "Unsafe use of new static()" errors.

-> https://phpstan.org/blog/solving-phpstan-error-unsafe-usage-of-new-static

[solverat]

All DocumentElementType\* classes should be declared as final.

I also don't think that this is a BC break since there is currently no way to extend / modify them.