message: Add Help link to .NET SDK install prompt
  This adds a Help link to the .NET SDK install prompt to help users troubleshoot problems with locating `dotnet`.
diff: @@ -85,16 +85,19 @@ function showDotnetToolsWarning(message: string): void {
  if (!config.get('suppressDotnetInstallWarning', false)) {
  const getDotNetMessage = 'Get the .NET Core SDK';
  const goToSettingsMessage = 'Disable this message in user settings';
  + const helpMessage = 'Help';
  // Buttons are shown in right-to-left order, with a close button to the right of everything;
  // getDotNetMessage will be the first button, then goToSettingsMessage, then the close button.
  vscode.window.showErrorMessage(message,
  - goToSettingsMessage, getDotNetMessage).then(value => {
  + goToSettingsMessage, getDotNetMessage, helpMessage).then(value => {
  if (value === getDotNetMessage) {
  let dotnetcoreURL = 'https://dot.net/core-sdk-vscode';
  -
  vscode.env.openExternal(vscode.Uri.parse(dotnetcoreURL));
  } else if (value === goToSettingsMessage) {
  vscode.commands.executeCommand('workbench.action.openGlobalSettings');
  + } else if (value == helpMessage) {
  + let helpURL = 'https://aka.ms/VSCode-CS-DotnetNotFoundHelp';
  + vscode.env.openExternal(vscode.Uri.parse(helpURL));
  }
  });
  }
  