Add VS Code Integration Tests

[agilgur5]

To ensure that TSDX "plays nicely" with VS Code out-of-the-box. Follow-up to my issues with reproducing #84 / #225 in #646

Also because this is both very difficult and very time-consuming to test manually due to having to load a full application (VS Code), using it/extensions, and resetting any user settings to make sure they don't interact with it.

Initial Proposal

Right now I'm thinking just tests for type-checking and linting (with vscode-eslint), but could add more in the future, like for testing (e.g. with vscode-jest).

I'm not totally sure how to do this right now is the problem. I think we can install VS Code, then any extensions, and then load the instance and programmatically run a "Command", and then check if the "Problems" pane correctly has output (against problematic code).

Any help appreciated with this