Add Python-like input() function to stdlib_io

[utsav-pal]

Implements a Python-like input() function as requested in #259.

Changes

• Added input() function to stdlib_io module
• Function provides convenient user input with optional prompt
• Uses existing get_line() function internally

Example Usage

use stdlib_io, only: input
character(len=:), allocatable :: name

name = input('Enter your name: ')
print *, 'Hello, ', name

Testing

• Full stdlib builds successfully (2049 files)
• All existing tests pass (407/407)
• Added unit test and example program

Files Changed

• src/io/stdlib_io.fypp - Core implementation
• doc/specs/stdlib_io.md - Documentation
• example/io/example_input.f90 - Usage example
• test/io/test_input.f90 - Unit test

Related Issue

Closes #259