Allow use of ExcelReference from outside Excel context

[govert]

From this discussion: https://groups.google.com/g/exceldna/c/1CkGn4zLA18/m/0S9ocPxvAAAJ

For testing purposes, it can be convenient to use the ExcelReference class in a context where the code is not running as an Excel add-in in Excel. This scenario broke in version 1.1 and since has caused a NullReferenceException to be thrown.

There is a workaround for this under .NET Framework, but it might be useful to create some public and supported way to do this more generally.