Support find widget in WebviewViewProvider

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> VSCode already has an options(https://github.com/microsoft/vscode/blob/main/src/vscode-dts/vscode.d.ts#L8620) to enable find widget in webview panel, but seems there is no way to enable it via webviewViewProvider(https://github.com/microsoft/vscode/blob/main/src/vscode-dts/vscode.d.ts#L8892) Hope VSCode can provide an option to enable find widget for WebviewViewProvider!