Consider abstracting some parts out of JavaScript / CSS build tools

Problem/Motivation

#3060153: Proposal to use PostCSS for Claro in core added build tools for CSS that are similar to JavaScript. We might be able to abstract some of the tooling out of these tools to reduce duplication.

Proposed resolution

-

Remaining tasks

Decide whether we want to introduce more complexity to the code by abstracting out the code. Maybe some of it should be moved to an external library as part of #2957390: Use ES6 for contrib and custom JavaScript development.

User interface changes

API changes

Data model changes

Release notes snippet