Add CSS breakpoint for modal.

[hammad-521]

Description

Added CSS variables for modal width breakpoints and refactored the modal-test.html file to demonstrate different modal sizes.

Motivation & Context

This change allows users to set different breakpoints for different modals using CSS variables, rather than just globally with SCSS variables. It provides more flexibility in customizing modal sizes across different use cases.

Type of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [x] Refactoring (non-breaking change)
• [ ] Breaking change (fix or feature that would change existing functionality)

Checklist

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

Live previews

• https://deploy-preview-40787--twbs-bootstrap.netlify.app/
• https://deploy-preview-40787--twbs-bootstrap.netlify.app/docs/5.3/components/modal/#custom-properties-for-modal-sizes