⚡ Improve slice allocation efficiency in casing wrappers

[arran4]

💡 What: Optimized ToKebabCase, ToSnakeCase, ToPascalCase, and ToCamelCase to use a single slice allocation strategy (make + append) instead of the inefficient double append pattern.

🎯 Why: The previous implementation used append([]Option(nil), append(opts, ...)...), which caused multiple allocations per call. This was redundant and inefficient.

📊 Measured Improvement:
Benchmarks show a consistent reduction of 1 allocation per operation across all measured scenarios.

• Baseline: ~8-15 allocs/op (depending on options)
• Optimized: ~7-14 allocs/op
• Latency: ~7% reduction in execution time (e.g., from ~561ns to ~520ns in basic case).

Verified that functionality remains unchanged with existing tests. Added benchmark_test.go to track performance.

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PR created automatically by Jules for task 17502699394544383921 started by @arran4

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[arran4]

lgtm