Add calculator

[skepttalk]

This PR adds a Calculator.java program to the Java-Programs repository.

The program allows users to perform basic arithmetic operations (+, -, *, /) on two numbers.
It introduces beginners to user input handling, switch statements, and basic arithmetic operations in Java.

[IamBisrutPyne]

Hey @skepttalk

Basically reiterating what I said in your previous PRs:
These are great contributions to the Beginner folders.
As I have stated in the guidelines,

Please only submit one program per PR to maintain brevity and ensure mergeability for Hacktoberfest

As noted in your last PR, administrative files like CONTRIBUTING.md are centrally managed and procedurally-generated. Please remove the CONTRIBUTING.md file from this PR before we can review the code.
Also, the file is missing the required Javadocs-style header block (Title, Author, Date, Description, Time Complexity, and Space Complexity). (Note: This simple solution is $O(N)$ Time and $O(N)$ Space).