Allow `<AddInName>_IntelliSense` folder to search for `...IntelliSense.xml` files

Maybe instead in allowing `XInclude` (see issue #81) you could allow a (sub)folder `_IntelliSense` in which several `.IntelliSense.xml` files can be stored to allow e.g. having one XML file per module in the AddIn.

The only problem I see so far is, what happens if there is both, the `.IntelliSense.xml` file _and_ the `_IntelliSense` folder. Give one of them priority or try to load both?

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_My goal is to use the XML files for IntelliSense as well as to use it to register the functions (i.e. use the content of the XML file for `Application.MacroOptions`)._

_Of course I am willing to share the result. There is already a working prototype. After I have added some unit tests (in VBA with the help of the awesome [Rubberduck AddIn](https://rubberduckvba.com/) I could contact you, so you can have a look, if you are interested. If yes, how do you prefer that I contact you?_