Add size choice for modal

Per [docs](https://getbootstrap.com/docs/4.5/components/modal/#optional-sizes), modals can be sized as small, default, large, and extra large via choice of class on the `.modal-dialog` element

It would be handy if this was exposed in the editor

No file or test is named. Start with the linked Bootstrap 4.5 modal documentation and trace how the editor generates the `.modal-dialog` element; done means the editor exposes small, default, large, and extra-large size choices with the corresponding modal classes.

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