ETK: Improve build scripts

Overview

The editing toolkit plugin build scripts are kind of convoluted. This is difficult for new contributors having to add a significant amount of boilerplate which is the same for all submodules, but also results in very strange build output. Instead of one single JS/CSS bundle which is loaded and sent to clients, we create different assets for every single submodule (i.e. about 20 different script files.

Additionally, the final plugin artifact includes the unminified source code. This means that diff creation and patching is much bigger than it needs to be.

This is probably bad for performance. However, one benefit of this approach is that we can decide to load files on a granular level. For example, since dotcom-fse is rarely used, its assets are almost never loaded. On top of that, folks have been using PHP to feature gate most of their features. For example, global styles requires theme support, and only then will its assets be loaded.

@josephscott what do you think about this issue? How do we balance not loading code on a granular level vs loading multiple files? Is it generally better to load one larger file, or multiple files which, in total, are smaller?

also cc @jsnajdr @sirreal for feedback

Context

• n/a

Start by reading apps/editing-toolkit/package.json and inspecting its current build scripts and generated plugin artifact. Compare the per-submodule assets and bundled artifact, then establish the desired loading and source-inclusion behavior with maintainers; done is not defined until that tradeoff is decided.