refactor(schema): restructure API from builder pattern to database-centric design

- Rename SchemaDialect to DatabaseDialect with comprehensive documentation
- Replace SchemaBuilder with Database struct containing schemas HashMap
- Restructure SchemaDef into Schema with direct table access
- Rename constraint types: TableDef→Table, ColumnDef→Column, etc.
- Add comprehensive documentation and examples for all public types
- Implement Default trait for DatabaseDialect and core structs
- Add utility methods: qualified_name(), has_primary_key(), etc.
- Expand test coverage with 50+ new test cases for all components
- Change parse_sql to mutate Database instead of returning Schema

BREAKING CHANGE: Public API completely restructured - SchemaBuilder removed,
all type names changed, and parse_sql method signature modified

Signed-off-by: Svetlin Ralchev <iamralch@users.noreply.github.com>

Author: iamralch