Add more tab=completion rules for DROP PROPERTY GRAPH

Here's a patch to add another tab-completion rule for DROP PROPERTY GRAPH [1]

The logic is the same as DROP MATERIALIZED VIEW [2] which has a very
similar syntax.

PSA v1.

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[1] https://www.postgresql.org/docs/19/sql-drop-property-graph.html
[2] https://www.postgresql.org/docs/19/sql-dropmaterializedview.html

Kind Regards,
Peter Smith.
Fujitsu Australia