How to pass data back from data to external_invoke()

Hello,

I was previously using webview and am looking into a rust binding. In my previous application I was able to pass a response back to my javascript using the `webview_return()` api. The javascript would then see the response as a Promise. So the equivlent would be to allow the return type of invoke_handler to be a string that would then be parsed on the javascript side.

Is this not supported feature?

Start by tracing the existing data, external_invoke(), invoke_handler, and webview_return() APIs described in the issue, then compare how responses are exposed to JavaScript. Determine whether returning a string or Promise is currently supported; done requires a maintainer decision on the supported behavior and its API scope.

Written by the indexing model from the issue text.