Dev: Pipe stdin from parent to app process

[RaeesBhatti]

- Summary
Pipe stdin of parent process to stdin of spawned app process.
Ref: #818 (comment)

- Test plan

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[ehmicky]

This should work in most cases.

However one issue is that the child process stdin will not be considered a tty anymore (process.stdin.isTTY !== true inside child process). This creates many issues with terminal utilities.

The only fix for it is to use stdio: inherit instead of stdio: pipe.

[RaeesBhatti]

We're actually hoping that child processes will not consider it a tty because that will prevents some frameworks from clearing the screen, which is problematic because it removes useful information logged by Netlify CLI. #374