feat: modernize virtual schema infrastructure

- Add Schema.get_table() for direct table access
- Add Schema.__getitem__ for bracket notation: schema['TableName']
- Add Schema.__iter__ to iterate over all tables
- Add Schema.__contains__ for 'TableName' in schema
- Add dj.virtual_schema() as clean entry point
- Remove create_virtual_module (breaking change)
- Fix gc.py to use get_table() instead of spawn_table()
- Remove specs/ folder (moved to datajoint-docs)
- Add comprehensive tests for virtual schema infrastructure

Fixes #1307

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: dimitri-yatsenko