[Feature Request] support system.stdin for interactive user input

[GeauxEric]

Review Mojo's priorities

• [X] I have read the roadmap and priorities and I believe this request falls within the priorities.

What is your request?

similar to Python's sys.stdin: https://docs.python.org/3/library/sys.html#sys.stdin

What is your motivation for this change?

Current applications can only handle user input either from file or from argv. If one wants to develop an app that takes interactive user input, such as a repr, there is no way.

Any other details?

No response