add PowerShell examples

[kshahar]

I was looking for an example of how to use websocketd with PowerShell on Windows. The provided .cmd files should be a good starting point. Tested on Windows 10 (PowerShell 5.1) and Ubuntu 18.04 (PowerShell 7.2).

Note: dump-env.ps1 uses GetEnvironmentVariables() from .NET instead of the built-in $Env: drive to potentially support different scopes (process / user / machine) if needed.