Add TypeScript 'abstract classes' guide. (#29514)

* Add TypeScript 'abstract classes' guide.

* fix/add-suggested-changes

guide/english/typescript/abstract-classes/index.md

@@ -0,0 +1,44 @@
+---
+title: Abstract Classes
+---
+
+# Abstract Classes
+
+Abstract classes are super classes which can be derived by other classes.The class marked with the `abstract` keyword is the abstract class and it can't be instantiate directly. The Abstract class may contain `abstract` or `non-abstract` member functions.
+
+```typescript
+abstract class FCC{
+    abstract makeSkill(): void;
+    getSound():void{
+        console.log("Coding...");
+    }
+}
+```
+
+Methods enclosed to abstract class which are marked `abstract` do not contain a method body and must be implemented in the derived class.
+
+```typescript
+abstract class FCC{
+    private name:string;
+    constructor(name:string){
+        this.name = name;
+    }
+    abstract makeSkill(skillName:string):void;
+    displayName():void{
+        console.log(`Hi, ${this.name} welcome to FreeCodeCamp!`);
+    }
+}
+
+class Camper extends FCC{
+    constructor(name:string){
+        super(name);
+    }
+    makeSkill(skillName:string):void{
+        console.log(`Hey, ${this.name} you made the ${skillName} skills`);
+    }
+}
+
+let jack = new Camper('Jack Smith');
+jack.displayName();
+jack.makeSkill('Front-End Library');
+```