Adding pipe_cin to examples dir

[wmarini]

@flagarde That pull request addresses the use of an unnamed pipe to read input stream in a program that uses the cpp_terminal library, as mentioned in issue #320.
It introduces a new example in the examples directory called cin_pipe. For my initial tests, I executed the following commands:

When the example program is executed without any unnamed pipe:

$ echo -e "Testing std::cin\nTesting stdin" | build/debug/examples/pipe_cin Checking if std::cin is empty
std::cin => "Testing std::cin"
Checking if stdin is empty
stdin    => "Testing stdin"

When the example program is the consumer of a unnamed pipe from a cat command:

$ echo "testing std::cin" | build/debug/examples/pipe_cin 
Checking if std::cin is empty
std::cin => "testing std::cin"
Checking if stdin is empty
stdin    => ""

$ echo -e "Testing std::cin\nTesting stdin" | build/debug/examples/pipe_cin Checking if std::cin is empty
std::cin => "Testing std::cin"
Checking if stdin is empty
stdin    => "Testing stdin"

Please verify the program in Windows and Mac environments, as it has only been tested on Linux Ubuntu 22.04 (xterm). Note that it uses only POSIX and C++11 flavors, so I believe it should work in any environment.

[flagarde]

@wmarini Thx for the code. unfortunately it seems windows is not totallt POSIX compatible but we can use your code as basis to do one with windows support.