Add a generic file watcher function in HelpersCommon.psm1

See https://github.com/PowerShell/PowerShell/pull/4523#discussion_r131796738

Polling for a file can be a common pattern. Consider implementing a generic file watcher function in HelpersCommon.psm1.
```
while (((Get-Date) - $startTime).TotalSeconds -lt 10 -and (-not (Test-Path "$HOME/nativeCommandProcessor.Success"))) {
    Start-Sleep -Milliseconds 100
}
```