Fix support for latest versions of Yeoman in VSCode extension build

[swernli]

The latest versions of the Yeoman packages types/yeoman-environment and types/yeoman-generator had breaking changes that caused our VSCode extension build to fail. We worked around this by setting fixed version numbers in #547 but we need to follow up with a real fix that ensures we can support their latest version.
@ricardo-espinoza for FYI.

[swernli]

As long as we are digging into this, perhaps we should consider moving off of the VSCode extension types that were deprecated a year ago. This warning comes up the first time you build the extension on a new repo:
npm WARN deprecated vscode@1.1.37: This package is deprecated in favor of @types/vscode and vscode-test. For more information please read: https://code.visualstudio.com/updates/v1_36#_splitting-vscode-package-into-typesvscode-and-vscodetest

[jasonNXP]

Any updates on this yeoman-environment update? I've managed to get my extension to build without errors, but now I'm struggling to get the extension to run in a debug environment. When attempting to load the extension with yeoman-environment included, I receive an error while activating, claiming it cannot find my extension's package.json.

[edit] Still in the process of learning the ins and outs of Javascript and the associated tooling. Turns out the webpack doesn't do so well with dynamic dependencies, so the errors I was encountering were related to webpack and the yeoman-environment dependencies. Moved to tsc, and the latest yeoman-environment is able to execute properly in vscode.