Review extended registration type support

[govert]

We're working on expanding the default function registration and merging the ExcelDna.Registration library into the main add-in library. The goal is for a larger set of functions to 'just work' without requiring extra registration steps.
Since we are no longer targeting older .NET Framework versions (< .NET Framework 4.5) we can incorporate Task<T> and other types into the main library.

Some of the current improvements are listed here: https://excel-dna.net/docs/guides-basic/extended-registration

I'm not sure what the right set of default type conversions are and realize that we're making a compatibility problem by returning object handles for all 'unknown' types.

I suggest we:

• Add support for string[] and string[,] return types, to match the parameter types
• Consider again whether the 'object handles' should catch all unknown types automatically. The problem is that once we return object handles for all unknown types, we cannot add automatic conversions for these types in future without breaking compatibility. Maybe we should reconsider whether a function should be explicitly marked to return / accept a handle?