Upgrade to ILSpyX from ILSpy 8

[christophwille]

With P3 https://github.com/icsharpcode/ILSpy/releases/tag/v8.0-preview3 the abstraction layer ILSpyX was added to nuget.org packages (for some details see Breaking Changes in the P1 announcements https://github.com/icsharpcode/ILSpy/releases/tag/v8.0-preview1). This should make a whole lot of ILSpy.Core "obsolete" in terms of using the actual, current code base from ILSpy matching the decompiler package (Note: the decompiler package is transitively referenced in ILSpyX, this means the dependency can and should be replaced).

[christophwille]

Other users of ILSpyX (so far, known):

• https://github.com/Open-Inventions/TurboSharp/blob/0a3433ba8496c48de576a501b019aa355bfe3137/src/Spy/TurboSpy.Lib/TurboSpy.Lib.csproj#L10

[christophwille]

The backend of ILSpy for VSCode is using ILSpyX: https://github.com/icsharpcode/ilspy-vscode/tree/master/backend/src/ILSpy.Backend

The general idea is to remove previously copied code and instead use it from ILSpyX (when available, of course)