Feature: C#-style extension method support

[TIHan]

C#-style extension method support:

public static class MyExtensions
{
    public static void MyExtensionMethod(this string str) { }
}

Can only be consumed explicitly at the moment:

main(): () =
    let str = "a string"
    MyExtensions.MyExtensionMethod(str)

We should add support that would look like this:

main(): () =
    let str = "a string"
    str.MyExtensionMethod()

Also, we need to consider adding support for C#14-style extension members:

public static class MyExtensions
{
    extension(string str)
    {
        // Method
        public void MyExtensionMethod() { }
        // Property
        string PlusOne => $"{str}1";
    }
}

Note: It's likely that once we handle C#-style extension methods, that C#14-style extension members will fit naturally.