Create ModularExponentiation.java

[lulunac27a]

Add modular exponentiation in Java

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[lulunac27a]

Add tests and code if y (power) is 0, return 1

[vil02]

I think you also want to throw an exception for y < 0.

[vil02]

@lulunac27a please fix all of the errors from the CI and please have a look at my previous comment.

[lulunac27a]

Fixed errors by formatting code and changing function name to modPow

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