Validate expressions passed to MarkLogic cts:search, etc.

[rhdunn]

MarkLogic places restrictions on the expressions passed to cts:search. Expressions that are not fully searchable raise XDMP-UNSEARCHABLE errors. It would be useful to validate these in the plugin.

Logic:

1. Locate function calls to functions that use fully searchable expressions.
2. Locate the argument subquery for the expression parameter.
3. Walk the subquery PSI tree.
4. If a node in the tree does not implement XQueryConformance, it is unsearchable and an error is raised.
5. If the node does not conform to MarkLogicSearchableXPath, it is unsearchable and an error is raised.

Reference:

1. https://docs.marklogic.com/guide/performance/query_meters#id_17135