Update StartVS script

This change updates the StartVS script in the following ways:

* Ported batch file to a PowerShell script. The batch file is still
  there. It just runs the PowerShell script.
* Added a `-chooseVS` switch that displays a menu of Visual Studio
  instances that are installed and allows the user to pick one to
  launch.
* Added an `-includeRoslynDeps` switch that sets an environment variable
  that controls whether Roslyn dependencies are included with the
  Razor extension or not.

Author: DustinCampbell

startvs.cmd

@@ -1,27 +1,4 @@
-@ECHO OFF
-SETLOCAL
+@echo off
+setlocal
 
-:: This command launches a Visual Studio solution with environment variables required to use a local version of the .NET Core SDK.
-
-:: This tells .NET Core to use the same dotnet.exe that build scripts use
-SET DOTNET_ROOT=%~dp0.dotnet
-SET DOTNET_ROOT(x86)=%~dp0.dotnet\x86
-
-:: This tells .NET Core not to go looking for .NET Core in other places
-SET DOTNET_MULTILEVEL_LOOKUP=0
-
-:: Put our local dotnet.exe on PATH first so Visual Studio knows which one to use
-SET PATH=%DOTNET_ROOT%;%PATH%
-
-SET sln=%~1
-
-IF "%sln%"=="" (
-    SET sln=%~dp0\Razor.sln
-)
-
-IF NOT EXIST "%DOTNET_ROOT%\dotnet.exe" (
-    echo .NET Core has not yet been installed. Run `%~dp0restore.cmd` to install tools
-    exit /b 1
-)
-
-start "" "%sln%"
+powershell -ExecutionPolicy ByPass -NoProfile -Command "& '%~dp0startvs.ps1'" %*

startvs.ps1

@@ -0,0 +1,75 @@
+[CmdletBinding(PositionalBinding=$true)]
+Param(
+    [Parameter(
+        Position=0,
+        Mandatory=$false,
+        HelpMessage="Solution file to open. The default is 'Razor.sln'.")]
+    [string]$solutionFile=$null,
+
+    [Parameter(
+        Mandatory=$false,
+        HelpMessage="If specified, choose the Visual Studio version from a list before laucnhing. By default the newest and last installed Visual Studio instance will be launched.")]
+    [Switch]$chooseVS,
+
+    [Parameter(
+        Mandatory=$false,
+        HelpMessage="If specified, Roslyn dependencies will be included in the Razor extension when deployed.")]
+    [Switch]$includeRoslynDeps
+)
+
+if ($solutionFile -eq "") {
+    $solutionFile = "Razor.sln"
+}
+
+if ($includeRoslynDeps) {
+    # Setting this environment variable ensures that the MSBuild will see it when
+    # building from inside Visual Studio.
+    $env:IncludeRoslynDeps = $true
+}
+
+$dotnetPath = Join-Path (Get-Location) ".dotnet"
+
+# This tells .NET Core to use the same dotnet.exe that build scripts use
+$env:DOTNET_ROOT = $dotnetPath
+${env:DOTNET_ROOT(x86)} = Join-Path $dotnetPath "x86"
+
+# This tells .NET Core not to go looking for .NET Core in other places
+$env:DOTNET_MULTILEVEL_LOOKUP = 0
+
+# Put our local dotnet.exe on PATH first so Visual Studio knows which one to use
+$env:PATH = $env:DOTNET_ROOT + ";" + $env:PATH
+
+$vswhere = "${env:ProgramFiles(x86)}\Microsoft Visual Studio\Installer\vswhere.exe"
+
+if ($chooseVS) {
+    # Launch vswhere.exe to retrieve a list of installed Visual Studio instances
+    $vsInstallsJson = &$vswhere -prerelease -format json
+    $vsInstalls = $vsInstallsJson | ConvertFrom-Json
+
+    # Display a menu of Visual Studio instances to the user
+    Write-Host ""
+
+    $index = 1
+
+    foreach ($vsInstall in $vsInstalls) {
+        $channelId = $vsInstall.installedChannelId
+        $lastDotIndex = $channelId.LastIndexOf(".")
+        $channelName = $channelId.Substring($lastDotIndex + 1);
+
+        Write-Host "    $($index) - $($vsInstall.displayName) ($($vsInstall.installationVersion) - $($channelName))"
+        $index += 1
+    }
+
+    Write-Host ""
+    $choice = [int](Read-Host "Choose a Visual Studio version to launch")
+
+    $vsBasePath = $vsInstalls[$choice - 1].installationPath
+}
+else {
+    # Launch vswhere.exe to retrieve the newest, last installed Visual Studio instance
+    $vsBasePath = &$vswhere -prerelease -latest -property installationPath
+}
+
+$vsPath = Join-Path $vsBasePath "Common7\IDE\devenv.exe"
+
+Start-Process $vsPath -ArgumentList $solutionFile