built-in extension for loading web content

We should have a plugin that is designed to load web content and display it. Most probably this should look like [Muxy](https://github.com/bengreenier/overlayed/tree/master/src/app/plugin/Muxy) and Muxy should be a more specific version of it.

The general idea here is to author a plugin that surfaces some config to drive a https://electronjs.org/docs/api/webview-tag element.

Start by reading the existing plugin at src/app/plugin/Muxy and the Electron webview-tag documentation. Work out how the proposed general web-content plugin should expose configuration and how Muxy would become a specialized version. Done means the built-in plugin can load and display web content through the stated configuration approach.

Written by the indexing model from the issue text.