* eng: onboard to the extension test runner
Adds a `.vscode-test.js` file that uses the new extension test CLI to
run tests. Also, onboards the markdown-language-features as the first
built-in extension to use it.
With the `ms-vscode.extension-test-runner` extension installed, the
markdown-language-features' tests can be run and debugged easily in
the UI :)
* fixup
Use the plugin version of TS lint rather than the external version. Add the plugin as a local dev dependency and add the vscode extension that contributes the plugin as a recomended extension. This should enable the plugin when using the standard version of TS and when using the workspace version of TS