Shall we have `IKeyed` interface in M3?

[enikao]

In our M3, both Language and NamespacedEntity have a property key: String.
We could introduce a common interface HasKey in M3.

LIonWeb Java has such an interface. It helps with finding all keys in a language, and check their conformity with spec.

On 2023-06-30, we decided NOT to have such interface.
Any concrete implementation is free to chose HasKey (or any other variant) as a way to implement the M3.

[enikao]

On 2023-07-07, we decided to rename NamespacedEntity to IKeyed with a property key: String. Language implements IKeyed. This effectively establishes an interface HasKey, with slightly different name.

Rationale: IKeyed name is symmetric with the existing interface INamed (#86).
We remove the notion of namespace as element in M3 (#146).

Side effect: Everything inside M3 realizes IKeyed. In this sense, something like ILanguageMember might be more suitable. However, such a name would be too similar to LanguageEntity (#147).