cs readOnly with final warning

[lublak]

Currently you get a warning if you declare a variable with @:readOnly: Warning : @:readOnly variable declared without never setter modifier also if you make the variable to final.
If you look into the doc of c# you can see that it is possible to set a readonly variable in the constructor: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/readonly