[CodeGenerator] Add `Introduce read-only field`

[SlavaRa]

Haxe

class Foo {
    function new(foo<generator>) {}
}

expect result:

class Foo {
    function new(foo) {
        this.foo = foo;
    }
    public var foo(default, null);
}

ActionScript

class Foo {
    function Foo(foo<generator>) {}
}

expected result:

class Foo {
    function Foo(foo) {
        _foo = foo;
    }
    private var _foo;
    public function get foo() {
        return _foo;
    }
}