feat(Modal): Added static backdrop animation defined in Bootstrap 4.4

[kyletsang]

• Bug fix
• New feature
• Chore
• Breaking change
• There is an open issue which this change addresses
• I have read the CONTRIBUTING document.
• My commits follow the Git Commit Guidelines
• My code follows the code style of this project.
• My change requires a change to the documentation.
• • I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

An animation was added for static backdrop modals in Bootstrap 4.4. This can be demoed at the following url:
https://getbootstrap.com/docs/4.4/components/modal/#static-backdrop

In addition to this, I fixed the esc key from closing a modal when the backdrop is set to static as per docs:
https://getbootstrap.com/docs/4.4/components/modal/#options

Includes a modal-backdrop element. Alternatively, specify static for a backdrop which doesn't close the modal on click or on escape key press.

[ALEUT]

The behavior was changed since version 4.2.0: before I was able to close the modal using Esc button when keyboard={true} and backdrop="static", but now I can't. Is it expected?

[TheSharpieOne]

Looks like it should allow esc to close the modal when keyboard is true. I opened a new issue to track this. Thanks you.

Also for reference: twbs/bootstrap#29978