Toast should have an option to not show close button

[tgreen7]

Environment

• Package version(s): 4

Feature request

The toast https://blueprintjs.com/docs/#core/components/toast should have a prop to not show the close button. We are showing a toast for a loading progress that has a cancel button. Users should not be able to close the toast, they should just be able to cancel it. Having a prop would be nicer than adding ugly css targeting. Maybe call it noCloseButton

Examples

[adidahiya]

I'm open to a PR for this. It would be better to call the new prop isCloseButtonShown?: boolean with a default value of true, just like <Dialog>.

[ohcnivek]

will work on this