Integrate core xml.xsd into the LinqToXsdCore library

XML.xsd are very commonly referenced in other schemas, such as http://docs.oasis-open.org/legaldocml/akn-core/v1.0/os/part2-specs/schemas/xml.xsd

Including them as built-in schema mapping in the library would obviate the need for client code to be generated and used by downstream consumers.