LinkedQL: Find Literals by Datatype

**Description**
Add the ability to find literals by their datatype (for instance all the Date literals in the graph).

The issue names LinkedQL but no files or tests. Start by locating the LinkedQL query entry point and existing literal-matching behavior; done means queries can return literals filtered by datatype, such as Date. Add or run focused coverage for that behavior if the project provides it.

Written by the indexing model from the issue text.