Restructure and retarget parts of core for use in generating definitions from LibreOffice IDL

[zspitz]

In order to further the goal of generating Typescript definitions for the LibreOffice API, the core project should be split into two projects:

1. writer- takes a TSNamespaceSet and produces a single Typescript string, formatted and split into lines
   This project should target .NET Standard
2. tlibuilder - generates a TSNamespaceSet from one or more ActiveX type libraries
   This project should target .NET Desktop, because it reads information from the registry, and uses TLBINF32 to get type library information
   In addition, there should be a third project:
3. idlbuilder - generates a TSNamespaceSet from the LibreOffice IDL files (and hopefully from other variants). This should target .NET Standard.

• Split projects as above
• Retarget appropriate projects to .NET Standard (when .NET Standard 2.0 is stable)

[zspitz]

We can't target 4.7 because we're depending on MoreLINQ (morelinq/MoreLINQ#280), which is currently incompatible with 4.7.