XMLISVALID() schema validation function

With MDEV-37261 we have implemented the XML Datatype.

We should implement the validation function XMLISVALID(string), which only checks if the string has a single root, closed tags, and proper nesting.
The overloaded function XMLISVALID(string,xml_schema) should additionally validate the. string against a XML Schema definition.
The xml_schema of XMLISVALID is either the schema text or the URL of the already registered schema. In this task we only implement textual schema's. Schema registering is for MDEV-39125.
As the complete XMLSchema standard compliance is to
big for a single task, below are limitations for this implementation:

NOT IMPLEMENTED:

• Including / inheriting other schemas.
  tags:
  <schema> | <include>
  <schema> | <import>
  <schema> | <redefine>
  <schema> | <override>
• Namespace control
  attributes like:
  <schema> | targetNamespace
  <schema> | elementFormDefault
  <schema> | attributeFormDefault
  <schema> | blockDefault
  <schema> | finalDefault

• complexContent not validated

• some basic types are not validated (just passed whatever there is)
  <list>
  <union>
  anyURI
  QName
  NOTATION
  normalizedString
  language
  NMTOKEN

• type facets are not validated. Things like number is too big or the
  string is too long.
  things like 'minInclusive', 'whiteSpace', 'pattern', 'minLength' etc..

• support global <element> index tags:
  <key>
  <keyref>
  <unique>