Use box-shadow CSS vars instead of Sass vars in assets and variables

[julien-deramond]

Description

This PR uses --bs-box-shadow* CSS vars instead of $box-shadow* Sass vars in our scss/_variables.scss and our site/assets/scss/_*scss in the same spirit of what we can do with --bs-border-width for instance.

In order to test it for some use cases, don't forget to enable locally $enable-shadows to see the shadows, or to keep it disabled to check for non-regressions.

There shouldn't be anything to mention in the migration guide or that can be breaking for folks. It should be safe to be embedded even in a dot version.

Motivation & Context

Following up #38816 and the following comment #38816 (review).

Type of changes

• Refactoring (non-breaking change)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• (N/A) My change introduces changes to the documentation
• (N/A) I have updated the documentation accordingly
• (N/A) I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-38976--twbs-bootstrap.netlify.app/ (sidebar and navbar at different breakpoints)
• https://deploy-preview-38976--twbs-bootstrap.netlify.app/docs/5.3/forms/form-control/ (inputs)
• https://deploy-preview-38976--twbs-bootstrap.netlify.app/docs/5.3/forms/select/
• https://deploy-preview-38976--twbs-bootstrap.netlify.app/docs/5.3/forms/range/
• https://deploy-preview-38976--twbs-bootstrap.netlify.app/docs/5.3/components/dropdowns/
• https://deploy-preview-38976--twbs-bootstrap.netlify.app/docs/5.3/components/popovers/
• https://deploy-preview-38976--twbs-bootstrap.netlify.app/docs/5.3/components/modal/

[louismaximepiton]

It feels safer than border-width since we don't modify the CSS vars. LGTM!