add xxl size modal

[TommasoAllegretti]

Description

add xxl modifier for modal

Motivation & Context

as requested in issue #39433

Type of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] 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
• [ ] I have added tests to cover my changes
• [ ] All new and existing tests passed

Live previews

• https://deploy-preview-39477--twbs-bootstrap.netlify.app/docs/5.3/components/modal/#optional-sizes

Related issues

Closes #39433

[coliff]

Thanks for making this PR based on the issue I opened. :-)

If it's approved we'd need to update the Modal docs too: https://github.com/twbs/bootstrap/blob/v5.3.2/site/content/docs/5.3/components/modal.md#optional-sizes

[TommasoAllegretti]

In case this PR gets approved I went ahead and added the necessary documentation. Feedback is very welcome as this is my first contribution.