Formation Deprecation - Core import swap

Description

The last remaining step that needs to be done in order to remove the Formation package from vets-website is to replace the import path for core.scss.

This Formation stylesheet has a counterpart in css-library that brings imports the necessary styles vets-website relies on to maintain visual parity.

We need to replace the vets-website import path from Formation to css-library, ensure we've done visual regression testing in several high traffic applications, static pages, and the injected header, and then merge a PR into vets-website for this change.

Considerations

css-library pull request that includes necessary updates to stylesheets in order to maintain parity with what is in production.
vets-website pull request that updates the import path and adds grid classes to shame.scss
For now, grid classes need to continue to be maintained manually. We can either leave them in shame.scss for the time being, or bring them into css-library and import them through core. Either way, the classes need to exist as they currently do because they do not rely on !Important, unlike utility classes.
Testing needs to be done thoroughly on this ticket. Even though the lines changed are relatively small, the impact on vets-website is potentially high. Screenshots of each thing tested need to be attached to the vets-website PR listed above.
At a minimum we should test: Profile, MHV Secure Messaging, Combined Debt Portal, Claims Status, content-build hub pages, at least one form application
Content-build will inherit changes locally from vets-website as long as both repos are running locally at the same time.

Tasks

[ ] Local testing with verdaccio, attach screenshots for applications tested
[ ] Merge css-library PR and publish a new version so we can spin up a review instance in vets-website
[ ] Review instance testing in vets-website to corroborate local diff testing
[ ] Merge vets-website pull request

Acceptance Criteria

[ ] Above tasks have been completed

department-of-veterans-affairs / vets-design-system-documentation