Support coding guidelines for C# 9.0

[wcoder]

https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-9

Pattern matching enhancements

new

if (e is not null)
{
    // ...
}

or

if (e != null)
{
    // ...
}

• t is not > 4 or not < 10 and not 0 and 0

Fit and finish features

new

var a = CreateA();

// ...

public A CreateA()
{
    // ...

    return new() // <-- here
    {
        Str = "some text"
    };
}

or

public A CreateA()
{
    // ...

    return new A // <-- here
    {
        Str = "some text"
    };
}

---

new

StringBuilder sb = new();

or

var sb = new StringBuilder();

TODO...