Update notebook messaging APIs in webview

[connor4312]

• Remove messaging APIs from the renderer. These didn't work/didn't work well and we have directed authors to use kernel preloads for communication.
• Add an event and an event available as globals in the kernel for them to communicate:

declare global {
    const onDidReceiveKernelMessage: VSCodeEvent<any>;
    function postKernelMessage(data: unknown): void;
}

• Preloads can reference this by (update @types/vscode-notebook-renderer and then) adding "types": ["@types/vscode-notebook-renderer/preload"] to their tsconfig.json
• Breaking: removed the generic postMessage handler -- extensions should use postKernelMessage instead.

[connor4312]

Also, while I was there -- with our new way of dealing with preloads, we no longer need to have extensions pass their ID when calling the acquire API. We already know it.