Standardize prompt behavior

[oesgalha]

The CLI uses distinct keys to navigate through the options in the first prompt (asking which CI platforms to use) depending on the OS.
It uses arrow keys in Linux and mac, but uses "u" and "d" in Windows.

It looks like this is a known issue in Deno. For now this is stated through a hint in the CLI, but we should watch these issues:

• up down left right don't response c4spar/deno-cliffy#272
• Presses of special keys are not detectable by Deno on Windows denoland/deno#5945