Feat: + && - overload with tests

[ayaankhan98]

Description of Change

Operator + overload for largeInt + largeInt

Checklist

• [x] Added description of change
• [x] Proper file name given
• [x] Added tests and example, test must pass
• [ ] Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• [ ] Relevant documentation/comments is changed or added
• [x] PR title follows semantic
• [x] Search previous suggestions before making a new one, as yours may be a duplicate.
• [x] I acknowledge that all my contributions will be made under the project's license.

Notes:

[ayaankhan98]

Here is an overview of what got changed by this pull request:

Clones added
============
- big-int/src/operators/plus.cpp  1
- big-int/src/operators/minus.cpp  1

See the complete overview on Codacy