chore: add GlobalUsings.cs with project-wide using directives in data structures

[KaranChadha10]

Description

This pull request contains the following changes

• Converted block-scoped namespaces to file-scoped namespaces for consistency in Data Structures folder
• Removed redundant/unused global using directives to improve code clarity
• Aligned code formatting with project conventions and C# 10+ style guidelines

Issue Link: #520

• I have performed a self-review of my code
• My code follows the style guidelines of this project
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Comments in areas I changed are up to date
• I have added comments to hard-to-understand areas of my code
• I have made corresponding changes to the README.md

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.15%. Comparing base (767c45a) to head (282fb7a).

Files with missing lines	Patch %	Lines
...es/LinkedList/SinglyLinkedList/SinglyLinkedList.cs	50.00%	0 Missing and 2 partials ⚠️
DataStructures/RedBlackTree/RedBlackTree.cs	0.00%	0 Missing and 1 partial ⚠️
DataStructures/Tries/Trie.cs	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #525      +/-   ##
==========================================
- Coverage   95.18%   95.15%   -0.03%     
==========================================
  Files         272      272              
  Lines       10921    10921              
  Branches     1536     1536              
==========================================
- Hits        10395    10392       -3     
- Misses        400      403       +3     
  Partials      126      126              

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