Decouple features from the Application Shell

with #351 we have a partial separation of the Application/Shell from features. Features have been moved out of the sample app `app-vuu-example` and into separate packages, peers of `app-vuu-example`. There is currently only one feature = `filtered-grid` but more will be added.

Both Application Shell and features are currently processed by a single build (as multiple entrypoints) and we rely on the bundling and code splitting capabilities of esbuild to produce the necessary bundles.

Eventually, we want to take this decoupling further - with completely independent builds for features. This will need to utilise es6 module loading rather than a monolithic build.

The issue names no files, tests, or entry points. Start by mapping the current single build and its multiple entrypoints, then determine how the Application Shell and feature packages should be built and loaded independently. Done means features no longer depend on a monolithic build and use ES6 module loading.

Written by the indexing model from the issue text.