Add init Methods

[jetyb]

Hello. I suggest to add init Methods. They have modifier init and:

1. Can change readonly fields and properties.
2. They can be called only from constructors and another init Methods.
3. They can be only private.

class Example
{
       public readonly int field1;
       public readonly int field2;
       public int Property3 {get;}
       public Example() => Method1(0);
       public Example(int value) => Method1(value);
      
       private init void Method1(int value)
       {
             field1 = value;
             Property3 = value * 5;
             Method2();
        }
        private init void Method2() => field2 = field1 * 3;
}

I more like keyword readonly , but this keyword is already used in some method declarations. So i suggest keyword init or some another. I dont think it would be very hard task.
With this new feature we can have more refactoring in constructors code and can call same code that can modify readonly variables.

[TahirAhmadov]

#8546
https://github.com/dotnet/csharplang/blob/main/meetings/2020/LDM-2020-06-15.md#init-methods