beautify `print()` method

[brownag]

1. [ ] Simple way of listing registered gpkg_contents tables
2. [ ] Identify presence of standard/core tables (gpkg_contents, gpkg_spatial_ref_sys, gpkg_geometry_columns, etc.) as a separate list
3. [ ] Add symbols or some other notation for optional associated features (e.g. presence absence of rtrees associated with specific features, metadata tables, table relationships (?))

Item 3 intended as a way of using as markup to enhance item 1 rather than listing all (long, possibly numerous) names