fix (#1660): Toaster zIndex

[Kamal0078k]

Overview Changed default zIndex value of toaster to 100 and it can be changed as shown in below:

import { toaster } from 'evergreen-ui'
toaster.danger('Message', { zIndex: 140 })

Screenshots (if applicable)

Documentation

• [ ] Updated Typescript types and/or component PropTypes
• [ ] Added / modified component docs
• [ ] Added / modified Storybook stories

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