Add a Getter for Defined Names

[WilliamTCarroll]

One can now access the Defined Names by the given Name and Sheet.

A test has also been added to verify this operates as expected.

Should there also be a function to get all of them for external looping? My use case does not need this, but someone might want it.

[MortalreminderPT]

Thanks for the pr, I'll check and merge it later! 😊