Recent versions of mdarea are quite powerful, and have internal state data structures. As a result, updating current editor value via `oninput` event listeners isn't reliable. Instead, we can use the mdarea extension API to handle keypresses exactly when markdownarea wants us to.
This also removes the `super` call from `build`, and directly creates/configures the applicable DOM elements. Since the mdarea editor driver's initialization is already quite different from BasicEditorDriver, it should fully own this step.