Create toast component

Via gitlab-design#65 (closed) we have added a new toast UI component to display short system messages in our pattern library and Design System docs.

• Pattern library issue: gitlab-design#65 (closed)
• Design System documentation: https://design.gitlab.com/components/toasts

Since more and more planned features are awaiting this addition and it greatly enhances the user experience compared to our existing flash messages, let's create this toast component within gitlab-ui.

They have also been part of the top 10 components list provided by the UX team.