Add FunctionParameterAssignment sniff

[aik099]

Summary

• Adds CodingStandard.CodeAnalysis.FunctionParameterAssignment, which warns when a function/method parameter not passed by reference is reassigned inside the function body — code reading the parameter afterwards would otherwise see the last-assigned value instead of the original argument.
• Reports one warning per assignment occurrence (not just the first), so all offending lines are visible for a given parameter.
• Abstract methods and interface methods (no body) are skipped, since there's nothing to analyze.

Test plan

• vendor/bin/phpunit CodingStandard/Tests/CodeAnalysis/FunctionParameterAssignmentUnitTest.php
• vendor/bin/phpunit (full suite, 30/30 passing)

Closes #104

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.72%. Comparing base (1ba8f6b) to head (3bb353d).

Files with missing lines	Patch %	Lines
.../CodeAnalysis/FunctionParameterAssignmentSniff.php	96.42%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #117      +/-   ##
============================================
- Coverage     97.74%   97.72%   -0.03%     
- Complexity      502      513      +11     
============================================
  Files            29       30       +1     
  Lines          1465     1493      +28     
============================================
+ Hits           1432     1459      +27     
- Misses           33       34       +1     

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