Add column types design document

Analyzes three design options for how Datasette could support column types
(datetime, file references, URLs, etc.) that cross-cut rendering, editing,
validation, and faceting. Recommends a hybrid approach: use the existing
metadata_columns system as the foundation with a "type" metadata key
convention, and optionally layer a ColumnType plugin registry on top for
plugins that want to provide a full bundled type experience.

https://claude.ai/code/session_01TitjhUZrRR4hzS3V5uoQoi