Linked list methods

[AmirReza-alt]

Add some usefull methods to LinkedList data structure.

List usually have some methods and properties that let user to give some information from the list like length.
I add some of this methods to the class to make it better.
This is a list of what methods i add:

• indexOf()
  to find a index by it's value.

• at()
  to find a value by it's index.

• print()
  to print the entire list. example : 30 ==> 40 ==> 50 ==> null

• getSize()
  to get the length of the linked list.
  its better to define the property for this one but i prefere to don't touch the entire code and i just make a method for this one.

I hope it helps. This is my first contribution. If it's not a problem for you please tell me my mistakes to make myself better.😊