Add a PR validation check to verify changed playlists are compliant to the schema

See #7181 for an example of what happens when playlist.xml files are changed and are no longer valid against the xsd file. We can add some verification when playlist.xml files are changed to ensure they remain valid to the schema.

Read issue #7181 and locate the changed playlist.xml files together with the referenced XSD schema. Then inspect the repository's PR validation entry point and determine how schema checks are run; done means a PR changing playlist.xml fails validation when the file does not comply with the XSD.

Written by the indexing model from the issue text.