[Data Liberation] New "wp data-liberation" WP-CLI script

[zaerl]

Motivation for the change, related issues

This code is cherry-picked from #2030. This is done to publish those parts during this discussion: #2090.

Implementation details

This PR also introduces the new CLI script and moves the logger there. I removed the old version and moved to a new standard class. The Logger class was removed because it is no longer needed now that the WP_Entity_Importer class exposes all the necessary information.

Testing Instructions (or ideally a Blueprint)

wp data-liberation import your-file.xml

or:

cd packages/playground/data-liberation/bin/import
bash import-wxr.sh a-folder-with-xmls

Run wp data-liberation import --help for all the options.