Loads the ./WebView2Loader.dll for running on Windows. Removes old version
if it already existed, and only runs once. Should be run on the main thread
so that the unload gets hooked in properly, otherwise make sure unload
gets called during the window.onunload event (after all windows are
closed).
Unload the library and destroy all webview instances. Should only be run
once all windows are closed. If preload was called in the main thread,
this will automatically be called during the window.onunload event;
otherwise, you may have to call this manually.