Add an on snippet event to psEditor API

It'd be nice to have an event we could hook into when a snippet is used. It would be great if the
the ast of the snippet, the variables, and maybe snippet metadata like name was passed to the handler.
Alternatively (or additionally) allow editor commands to act like snippets (e.g. set "variables" to
tab to, etc.) This would enable dynamic snippets that can expand or shape the snippet based on command
output.

A very simple mock up:

/* VSCode snippet example */
"Comment Help with PowerShell Output": {
    "prefix": "CommentHelp",
    "body": [
        "<#",
        ".SYNOPSIS",
        "    ${Short description}",
        ".DESCRIPTION",
        "    ${Long description}",
        ".NOTES",
        "    Date: ${PowerShell1:Get-Date -Format MM-dd-yyyy}",
        ".LINK",
        "    ${PowerShell2:Get-Command -Module \\$MyInvocation.MyCommand.Module.Name}",
        "#>"
    ]
}

# PowerShell event handler.
$eventHandler = {
    param($snippetVariables)
    foreach ($variable in $snippetVariables) {
        if ($variable.Name.StartsWith('PowerShell')) {
            $script = [scriptblock]::Create($variable.Value)
            $output = & $script
            $psEditor.GetEditorContext().CurrentFile.InsertText($output, $variable.Range)
        }
    }
}

<#
.SYNOPSIS
    Short description
.DESCRIPTION
    Long description
.NOTES
    Date: 04-12-2017
.LINK
    Get-Function1
    Get-Function2
    Get-Function3
#>

La issue non indica file, test o punti di ingresso dell’implementazione. Inizia individuando l’API di psEditor e il percorso esistente per la gestione degli snippet, quindi chiarisci se l’obiettivo è un evento, snippet basati su comandi o entrambi. Il lavoro completato dovrebbe includere un contratto di handler definito per snippet AST, variabili e metadati, con comportamento e test che coprano l’ambito scelto.