Create code fix to convert non-nullable reference type auto property to null safe version

[Keboo]

We would like to create a code fix to convert auto properties to null safe equivalent following the pattern in #88.

This code fix should not be shown for:

• Read only properties
• Nullable reference types
• When nullable is not enabled

Expected:
This

public string FirstName { get; set; }

Becomes:

//TODO: Initialize FirstName during initialization
private string? _FirstName;
public string FirstName
{
    get => _FirstName!;
    set => _FirstName = value ?? throw new ArgumentNullException(nameof(value));
}

This:

public string FirstName { get; private set; }

Becomes:

//TODO: Initialize FirstName during initialization
private string? _FirstName;
public string FirstName
{
    get => _FirstName!;
    private set => _FirstName = value ?? throw new ArgumentNullException(nameof(value));
}