Sub-theme custom JS assets are always combined into script.js

Problem/Motivation

CivicTheme's sub-theme needs 2 ways of working with JS files provided by the sub-theme:

1. Bake additional JS files into dist/script.js
2. Minifiy and export library JS files into dist/[name].js

This needs to be done in such a way that any developer can understand the difference between the two.

Steps to reproduce

1. Add `assets/custom.js` to sub-theme
2. `npm run dist`
3. Observe the contents of `custonm.js` in `dist/scripts.js`

Proposed resolution

None

Remaining tasks

None

User interface changes

None

API changes

None

Data model changes

None