Add additional information about classes

[deyantomov]

I want to add the following topics to the section about classes:

1. Getters and setters
2. Class composition ("has-a" relationship, instead of the "is-a" relationship that inheritance creates)
3. Method overriding

[sumn2u]

I want to add the following topics to the section about classes:

1. Getters and setters
2. Class composition ("has-a" relationship, instead of the "is-a" relationship that inheritance creates)
3. Method overriding

@deyantomov Some examples of Getters and Setters are introduced in following topics.

• Understanding Getters and Setters in JavaScript
• Getters and Setters

[deyantomov]

I want to add the following topics to the section about classes:

1. Getters and setters
2. Class composition ("has-a" relationship, instead of the "is-a" relationship that inheritance creates)
3. Method overriding

@deyantomov Some examples of Getters and Setters are introduced in following topics.

* [Understanding Getters and Setters in JavaScript](https://github.com/sumn2u/learn-javascript/blob/main/en/functions/getters-setters.md)

* [Getters and Setters](https://github.com/sumn2u/learn-javascript/blob/main/en/miscellaneous/Added%20getters%20and%20setters.md)

I see, I must've been looking for the topics in the wrong place. I'll go through my notes and get started on composition then. I'm not 100% certain about method overriding, as the topic is not that broad, but I'll see what I can come up with.