Exclude main.min.css from preprocessing and minification

Problem/Motivation

main.min.css is 48kb and pre-minified.

When visiting multiple different pages in Drupal CMS, CSS from components and similar gets conditionally added to CSS aggregates. This means that main.min.css can end up being included in multiple different aggregates.

You can see this in #3554115: Add performance tests where over 2mb of CSS is downloaded via visiting each of the pages in Byte's main menu.

See #734080: Set preprocess: false for jquery.min.js to reduce duplication between asset aggregates for a similar (js) example.

Steps to reproduce

Proposed resolution

The easiest way to prevent this is to exclude the biggest files from aggregation, this way they're only downloaded once (per browser) regardless of how many library combinations exist on a site.

Remaining tasks

User interface changes

API changes

Data model changes