Lodash: Refactor away from _.groupBy() from compileCSS()

[tyxla]

What?

This PR removes Lodash's _.groupBy() from compileCSS() in the style engine package.

Why?

Lodash is known to unnecessarily inflate the bundle size of packages, and in most cases, it can be replaced with native language functionality. See these for more information and rationale:

• Remove lodash dependency from '@wordpress/element' #16938 (comment)
• Remove lodash dependency from WordPress packages. #17025
• @wordpress/api-fetch package has lodash as a dependency #39495

How?

We're using Array.prototype.reduce() instead.

Testing Instructions

• Verify that as you move, change, duplicate or edit blocks, their custom styles still persist as expected.
• Verify all checks are green - the changes are well covered by unit tests.

[aristath]

LGTM 👍