fix: toaster types

[filippofilip95]

Hello ! :)

Overview
This PR fixes the Typescript type of the toaster instance in the following way:

1. Retype title argument in toaster. The title was typed as only a string but instead, it can support ReactNode. Type ReactNode is for title also specified in interface Toast.
2. Add function remove to the toaster. Function remove is exposed in the toaster however it doesn't exist in types. Function remove is also already used in the storybook here.

Screenshots (if applicable)
Documentation doesn't need a change since it's already considering that type title is a node.

Documentation

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

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[brandongregoryscott]

Thanks for the contribution, @filippofilip95! This should be fixed in https://github.com/segmentio/evergreen/releases/tag/v7.1.4