feat(modal): add role option

[danilopiazza]

Before submitting a pull request, please make sure you have at least performed the following:

• read and followed the CONTRIBUTING.md and DEVELOPER.md guide.
• built and tested the changes locally.
• added/updated any applicable tests.
• added/updated any applicable API documentation.
• added/updated any applicable demos.

This PR should close #4101.

[danilopiazza]

I have a few remaining questions:

• API documentation: which version should I document in the @since tag for the new property?
• Demo: what should I showcase in the demo? Should I print the role attribute from the modal window, just to see if it is woking? It looks like I would be duplicating the end-to-end tests.
• Should the new role option be updatable, like ariaDescribedBy and ariaLabelledBy?
• At this time, the only possible values for role are alertdialog and dialog. Should the property be strictly typed, allowing these two values only, or should it be extensible, allowing any string value (like fullscreen and size)?