Create logic for PropertyXmlSyntax, refactor the PropertyDescriptor interface

Depends on the new ruleset schema

Deals with multi-valued properties natively without a delimiter

Allows us to remove the multi-valued property classes, having only one impl per property type

This can be plugged easily into the current progress about the implementation of
RulesetFactory for the new schema, but we'll have a bastard parser for a while.

See https://github.com/pmd/pmd/wiki/Property-framework-7-0-0#new-property-syntax-in-xml

Update the documentation generator to take PropertyXmlSyntax into account

ℹ️ Note: There is a branch by @oowekyala that might contain some of the tasks above: pmd/7.0.x...oowekyala:pmd:1432-property-framework-upgrade-pmd7

ℹ️ Note: The property rewrite/update is part of [core] Update properties framework #4380 . This PR contains the following two commits, which reverted a partial finished solution for the new property syntax (specifically using <seq> for multi valued properties):

• 84cba78
• 24257a0
• These two commits and be used as a guidance, but probably can't be just reverted, because they probably would create API incompatibilities.