Refactor ColumnType: register classes, return instances with config

- register_column_types() now returns classes instead of instances
- ColumnType.__init__ takes optional config=, baking it into the instance
- get_column_type() returns a ColumnType instance (or None) instead of a
  (name, config) tuple
- get_column_types() returns {col: ColumnType instance} instead of tuples
- Remove get_column_type_class() - no longer needed
- render_cell/validate/transform_value methods no longer take config arg;
  use self.config instead
- render_cell hook takes column_type (ColumnType or None) instead of
  column_type + column_type_config

https://claude.ai/code/session_01SvPEPqHgURTWESRp28pTC3

datasette/hookspecs.py

@@ -65,7 +65,6 @@ def render_cell(
     datasette,
     request,
     column_type,
-    column_type_config,
 ):
     """Customize rendering of HTML table cell values"""