implementing factory design pattern

[RgiriH]

this pull request solves the issue #126 ,

main features:

• As the current tool only supports leetcode , but there are many other good coding platforms were people actively participate and this design pattern will help the tool to expand to other platforms with minimal changes in current work flow.

• the application logic for getting required data from the leetcode platform is now modularised in a specific subclass named leetcode in platform/leetcode making it easy to understand.

• Support to new platforms can be add easily by implementing the subclass for different platforms in the folder platforms which extends the supper-class named platform with appropriate implementation of the functions defined in that supper-class based on the HTML structure of the platform and a single condition statement in builderFunction .

• this encourages new contributors to easily contribute to increase the usability of the tools

Pre testing

• I have tested the tool with different cases in leetcode and have made sure it works as expected with out any bugs or error , before making this pull request

[RgiriH]

@piyushgarg-dev Please review my code and consider merging it if you think it adds any value to the tool and to the future contributors.