Recommend adding .vscodeignore

[adamwatters]

Related to Recommend adding .vscodeignore

New extension developers might have extensions with lots of files even after bundling because they haven't added a .vscodeignore file (speaking from personal experience 🤣- my extension had 23409 separate files and took about a minute to install)

The message This extension consists of ${files.length} separate files. For performance reasons, you should bundle your extension: https://aka.ms/vscode-bundle-extension. could be updated to include something like You should also exclude unnecessary files by adding them to your .vscodeignore: https://code.visualstudio.com/api/working-with-extensions/publishing-extension#.vscodeignore