eng: onboard to the extension test runner (#195570)

* 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