Add input() convenience function to stdlib_io with documentation, tests, and example (Fixes #259)

[Brijesh-Thakkar]

Adds a minimal Python-like input(prompt, stat) function to stdlib_io. Reads a full line, optional prompt + optional stat, follows stdlib error semantics, no parsing. Includes docs, tests, example, and CMake updates. Small, safe, ergonomic addition aligned with stdlib goals. Ready for review.