feat: add Fenwick Tree implementation with tests

[tanjumul]

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Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new JavaScript files are placed inside an existing directory.
• All filenames should use the UpperCamelCase (PascalCase) style. There should be no spaces in filenames.
  Example: User个人资料.js is allowed but userprofile.js,Userprofile.js,user-个人资料.js,user个人资料.js are not
• All new algorithms have a URL in their comments that points to 维基pedia or another similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

Summary

This PR adds a Fenwick Tree (Binary Indexed Tree) implementation to the Data-Structures/Tree directory and the test files as well.

Features

• Point updates in O(log n)
• Prefix sum queries in O(log n)
• Range sum queries in O(log n)
• Build tree from existing array
• Get/Set operations

Testing

• Comprehensive unit tests included
• Edge cases covered (empty tree, single element, negative values)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.95%. Comparing base (5c39e87) to head (e4a046e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1897      +/-   ##
==========================================
+ Coverage   85.91%   85.95%   +0.03%     
==========================================
  Files         379      380       +1     
  Lines       19778    19834      +56     
  分支     3016     3028      +12     
==========================================
+ Hits        16993    17049      +56     
  Misses       2785     2785              

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