Add new scope name for configuration

@nedbat @cpennington This is the work on adding a named scope for configuration.
This class has too many responsibilities. Instead of baking all three properties (ordered field data, dict lookup, read-only) into a single class, I would recommend 3 classes: `FallbackFieldData` (this would read in order from a set of subordinate field datas, and would write to the first field data, or some other sensible behavior), `TypedLookup` (this would read from a dict based on the type of the block for the Scope.configuration field), and `ReadOnlyFieldData`, which already exists.

That way, it becomes clearer what each field data is supposed to do, and also allows others to compose them in different ways in the future (perhaps adding another level of lookup before falling back to the _default values, for instance).