Fix C# var regex

Summary: This patch fixes the C# var data type regex to match correctly on all var keywords (e.g. https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/implicitly-typed-local-variables)

Test Plan: Tested with C# code from https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/implicitly-typed-local-variables

Reviewers: Framework: Syntax Highlighting, dhaumann

Reviewed By: Framework: Syntax Highlighting, dhaumann

Subscribers: dhaumann, Frameworks, Framework: Syntax Highlighting

Tags: Framework: Syntax Highlighting, Frameworks

Differential Revision: https://phabricator.kde.org/D10286