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
#>

El issue no nombra archivos, tests ni puntos de entrada de la implementación. Empieza localizando la API de psEditor y la ruta existente para gestionar snippets, y después aclara si el objetivo es un evento, snippets respaldados por comandos o ambos. El trabajo debería incluir un contrato de handler definido para snippet AST, variables y metadatos, con comportamiento y tests que cubran el alcance elegido.