Added isogram utility class and unit tests

[JeevaRamanathan]

This PR implements isogram detection in Java to determine whether a word contains each letter exactly once, using two methods and added unit tests.
isIsogramByArray() – Uses a boolean array for constant space validation.
isIsogramByLength() – Uses a HashSet to compare unique character count.

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

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.70%. Comparing base (8d14b49) to head (624e880).

Files with missing lines	Patch %	Lines
...c/main/java/com/thealgorithms/strings/Isogram.java	95.45%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6594      +/-   ##
============================================
+ Coverage     75.66%   75.70%   +0.03%     
- Complexity     5742     5755      +13     
============================================
  Files           698      699       +1     
  Lines         19703    19725      +22     
  Branches       3820     3826       +6     
============================================
+ Hits          14909    14933      +24     
+ Misses         4215     4214       -1     
+ Partials        579      578       -1     

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[alxkm]

Looks good. Thanks for the contribution.