Use new strategy for ConsoleReadLine.ReadKeyAsync to fix macOS and Linux issues

This change uses a new strategy for providing a quasi-async
Console.ReadKey implementation so that key handling works better on Linux
and macOS.  This should eliminate issues where keys and escape sequences
were being echoed incorrectly.  It should also eliminate the issue where
the backspace key is unusable on these platforms.

Resolves PowerShell/vscode-powershell#533.
Resolves PowerShell/vscode-powershell#542.

Author: daviwil