Implement IPC messaging API

[chocolatkey]

When using the reader as an embedded element within another app, such as an accessibility authoring tool, it is useful to be able to externally control the reader, which may be embedded using an <iframe> or opened as a popup. This is possible using the postMessage web API. An example of this communication could look like:

const reader = document.getElementById("reader"); // Reader <iframe>
reader.contentWindow.postMessage({
    _playgroundIPC: 1,
    id: crypto.randomUUID(),
    key: "go",
    data: {
        locator: {
            href: "OPS/chapter_001.xhtml",
            type: "application/xhtml+xml",
            locations: {
                cssSelector: "body>section>p:last-child"
            }
        }
    }
}, "*");