Move scope string generation logic to the base class (#56085)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56085

Changelog: [Internal]

Moves the logic responsible for generating a string view of the scope's body to the base class to reduce duplication.

Similarly, moves the logic responsible for qualifying template arguments to the `Extensible` class.

Reviewed By: cipolleschi

Differential Revision: D96455746

fbshipit-source-id: 1d800c7992080e02831f61fee4aaa9cb31f8fc9a