stdlib_io: add Python-like input() function

[Brijesh-Thakkar]

This PR adds a Python-like input() function to stdlib_io.

Features:

• Reads a full line from standard input
• Optional prompt support
• Preserves trailing whitespace
• No implicit type conversion
• Integrates with existing get_line infrastructure

Documentation, examples, and tests are included.
All CMake and ctest checks pass locally.

Fixes #259

[Copilot]

Pull request overview

This PR adds a Python-like input() function to stdlib_io, providing a convenient way to read user input from standard input with an optional prompt. The implementation leverages the existing get_line infrastructure and follows similar patterns to other I/O functions in the module.

Key Changes

• New input() function that reads a line from standard input with optional prompt support
• Preserves trailing whitespace and returns an allocatable character string
• Includes optional error handling via iostat and iomsg parameters

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
src/stdlib_io.fypp	Adds the input_char function implementation and input interface, exports it in the public API
doc/specs/stdlib_io.md	Adds comprehensive documentation for the new input function including syntax, arguments, return value, and usage notes
example/io/example_input.f90	Provides a basic usage example of the input() function
test/io/test_input.f90	Adds a test case for the input() function to verify whitespace preservation

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[Brijesh-Thakkar]

@jvdp1 Heyy, Please review this pr