Get powershell path in a better way

[TylerLeonhardt]

This grabs the implementation from vscode-powershell which has a more fully featured way of discovering what PowerShell exe's are installed on the system.

This also supports the same setting as vscode-powershell:

"powershell.powerShellAdditionalExePaths": [
        {
            "exePath": "/Users/tyleonha/.powershell-daily/pwsh",
            "versionName": "PowerShell Daily"
        },
        {
            "exePath": "/Users/tyleonha/Code/PowerShell/PowerShell/src/powershell-unix/bin/Debug/netcoreapp3.1/osx-x64/publish/pwsh",
            "versionName": "PowerShell Local Build"
        }
],

"powershell.powerShellDefaultVersion": "PowerShell Daily"

I'd say you don't have to worry about reviewing src/client/platform.ts since that was already battle tested in vscode-powershell but review everything else :)

This also means coc-powershell will bring in PSES 2.1.0 which includes a ton of perf improvements and PSReadLine support in the integrated console.