Update `test:webview` script (#339)
Update `test:webview` script (#339)

Description of changes

In response to [this sample toolkit PR](https://github.com/microsoft/vscode-webview-ui-toolkit-samples/pull/61), update the `test:webview` script to install the `component-gallery` extension from it's new URL and direct testers to run a new command to open the webview ("Component Gallery: Show").
microsoft/vscode-webview-ui-toolkit,microsoft/vscode-webview-ui-toolkit,microsoft/vscode-webview-ui-toolkit