feat(12.2-02): add the datepicker field with server-side bounds and date list columns

- type: datepicker compiles the D-20 keys; format maps to displayFormat with WinterCMS's momentFormat table
- boot fails when the mode does not match the column's Go type (time.Time, lagoon.Date, lagoon.TimeOfDay)
- datepicker is a writable scalar field; minDate and maxDate are rechecked on save
- columns.yaml accepts type: date and type: time; Scanner/Valuer structs are columns, not relations
- conformance fixture carries date and datetime fields; README, forms and lists docs

modules/cabana/contracts.go

@@ -92,6 +92,9 @@ type CompiledController struct {
 	// files are the form's `type: fileupload` fields bound to the model's
 	// attach.Relation, keyed by field name.
 	files map[string]*compiledFile
+	// dates are the form's `type: datepicker` fields after the Go type
+	// check, keyed by field name.
+	dates map[string]*compiledDate
 }
 
 // Registry is the immutable controller map keyed by controller ID.