Queries: Add support for hetero atom neighbors count

Support for hetero atom neighbors count should be added for tautomer rules.

SMART should support next atom attributes:
> z - An atom attribute indicating a required number of hetero atom neighbors. If no numeric modifier is supplied, one or more hetero neighbors are required.
Z - An atom attribute indicating a required number of aliphatic hetero atom neighbors. If no numeric modifier is supplied, one or more hetero neighbors are required.

QueryMolecule class should be updated to store and verify this attributes.

Start by locating the QueryMolecule class and the tautomer-rule query handling mentioned in the issue. Trace how SMART atom attributes are parsed, stored, and verified, then inspect nearby tests if available. Done means supporting z and Z with optional numeric modifiers for hetero-atom neighbor counts.

Written by the indexing model from the issue text.