Transmit whole preference section rather than individual fields

[colin-grant-work]

We are now transmitting quite a few separate preferences into the webview, and we're doing so by fetching every value we're interested in separately and then combining them into an object.

We could make that code less verbose if we simply fetched all of the preferences associated with the plugin at once and transmitted that to the webview:

return vscode.workspace.getConfiguration().get(manifest.PACKAGE_NAME) as OurMessageInterface;

The downside would be coupling between the package.json declarations and our data formats. But there's already a lot of coupling, since we're just pulling out the fields declared in the package.json one by one anyway.

Originally posted by @colin-grant-work in #89 (comment)