C# Method Syntax format grammar recognition

_From @CCludts on January 3, 2019 5:47_

The method name is only recognized as `source.cs` instead of `storage.type.cs` when formatting function signatures in certain cases.


``` csharp
public class Hello {
// correct syntax highlighting
public Task<(string users, string missingUserIds, string unprocessedUserIds)> FetchUsers() {
return null;
}

// different formatting. breaks syntax highlighting
public Task<(
string users,
string missingUserIds,
string unprocessedUserIds)> FetchUsers() {
return null;
}
}
```
_Copied from original issue: Microsoft/vscode#65951_