Add trim functionality with tests and update package exports

[ayoubjemai]

🧹 Fix: Improve typings for trim, trimStart, trimEnd, trimLeft, and trimRight on String
This PR enhances the type definitions for string trimming methods by introducing compile-time evaluation of the trimmed result. The updated types provide more precise behavior when working with string literals.

❓ Why
This makes the trimming methods more type-safe and predictable when working with string literals, especially in utility-heavy codebases where precise typing can improve DX and static analysis.

Let me know if you'd like tests or further refinements — happy to collaborate!

✅ Changes

• Added a Whitespace type to define characters considered whitespace (" ", "\n", "\t").
• Implemented TrimLeft and TrimRight recursive utility types to trim leading and trailing whitespace at the type level.
• Updated the String interface to:
  • Type trim as TrimLeft<TrimRight<S>>
  • Type trimStart and trimLeft as TrimLeft<S>
  • Type trimEnd and trimRight as TrimRight<S>