The system/base library makes sites load unnecessary CSS on most pages

Problem/Motivation

The system/base library has CSS that is critical to the functionality of some UI elements such as tabledrags. Currently, system/base library is always attached but for example the tabledrags related CSS is not needed on most pages.

Proposed resolution

Deprecate system/base and move CSS files to smaller libraries that are specific to the functionality. For example, tabledrag.module.css would be moved to the tabledrag library.

Remaining tasks

1. Agree that risk of having more CSS combination variations is a good trade off over loading unnecessary CSS
2. Decide should we keep CSS related to files in core/misc in system module or should system module extend the tabledrag libraries by adding the CSS files

User interface changes

API changes

Data model changes

Release notes snippet