Allow public getter and private setter for properties

[miniak]

Example:

class Foobar {
    private _value = 0;

    public get value() {
        return this._value;
    }

    private set value(value: number) {
        console.log(`setting value to: ${value}`);
        this._value = value;
    }

    public doSomething() {
        this.value = this.value + 1;
    }
}

You can do that for example in Objective-C, where it's really useful:

Foobar.h

@interface Foobar : NSObject
@property (readonly, nonatomic) int value;
- (void)doSomething;
@end

Foobar.m

@interface Foobar () {
    int _value;
}
@property (readwrite, nonatomic) int value;
@end

@implementation Foobar

- (int)value {
    return _value;
}

- (void)setValue:(int)value {
    NSLog(@"setting value to: %d", value);
    _value = value;
}

- (void)doSomething {
    self.value = self.value + 1;
}

@end

[mhegazy]

Duplicate of #2845