JavaScript: Detect duplicate private class elements and invalid constructors

[matthiasblaesing]

The JS parser should invalid constructs such as:

• Multiple declarations of constructor:

    class T {
        constructor() {}
        constructor() {}
    }

• Private constructors:

    class T {
        #constructor() {}
    }

• Fields that are named constructor:

    class T {
        constructor = X
    }

• Generators named constructor:

    class T {
        *constructor() {}
    }

• Accessors named constructor:

    class S {
        get constructor() {}
    }
  
    class T {
        set constructor(value) {}
    }

Private element must be declared only once:

• Accessor pair should be accepted

    class T {
        get #X() {};
        set #X(value) {};
    }

• Reject plain field and and accessor

    class T {
        #X = 1;
        get #X() {};
    }

    class T {
        #X = 1;
        set #X() {};
    }

• Plain field and method should be rejected

    class T {
        #X;
        #X() {};
    }

• Duplicate method should be rejected

    class T {
        #X() {};
        #X() {};
    }

• Duplicate fields should be rejected

    class T {
        #X;
        #X;
    }

• Mixed static declarations should be rejected

    class T {
        get #X() {};
        static set #X(value) {};
    }

Closes: #8953

[matthiasblaesing]

Unless I get objects, I'll merge early next week.