Implemented the Mergesort algorithm and renamed the AllSortingAlgorithms.cpp file

[lavish-github]

• Implemented the merge sort algorithm in CPP folder
• Renamed the AllSortingAlgorithms.cpp file to AllSearchingAlgorithms.cpp file, because I think it is implementing all searching functions only

Fixes #67

[extinctsion]

https://user-images.githubusercontent.com/43691873/135965000-a3019b12-c089-4a61-bae3-81f915ec39a5.png Pls read the readme file again. Your PR will be excluded by system. Pls submit genuine PR.

[lavish-github]

Can I submit a leetcode question? If not, then what can be counted as a genuine PR for a newbie like me?