OptionaltargetWhere message events arrive. Defaults to globalThis (the webview's
window); tests substitute a fake.
OptionalvscodeThe object returned by acquireVsCodeApi().
Pass this whenever your webview already calls acquireVsCodeApi()
itself (for getState/setState, say): VS Code allows exactly one
call per session, and a second one throws. When omitted, the client
calls it for you.
Options for createWebviewRpcClient.