[Proposal] Default partial flag

[workgroupengineering]

Roslyn's source generators have become very popular, but most project and item templates do not have a partial modifier in the class declaration, forcing you to modify each class definitions every time or generate custom templates.

It would be handy if there was the possibility of setting a property in csproj that would make all classes without partial modifier considered partial.

[ufcpp]

#950
#3748