Implement communication channel between VS Code extension and webviews

Created by: tjkandala

VS Code extension can send messages to webviews, and vice versa, through postMessage() methods on their respective APIs. This is useful, but can quickly get out of hand as we find more reasons to communicate between the two contexts (like #26376). In addition, it's harder to unit test, let alone write, components that rely on messaging w/ the extension.

We should write a comlink adapter for VS Code extensions.