added dijkstra algorithm in cpp

[adarsh115]

Relates to #166

Description:

Checklist

<Put an x between square brackets wherever appropriate without any spaces like [x]>

• I've followed the Contributing guidelines provided in the repository.
• I've made the changes which were demanded in the linked issue.
• My code gave a clean output on debugging. (NO warnings/errors)

[adarsh115]

@Vishal19111999

[sanskarseth]

@adarsh115 please take the input from the user and then store it as an adjacency list or matrix so that this code can be useful in competitive programming.

Rest looks fine :)

[vishalvibes]

@adarsh115 don't use define, use const int instead