Add `Blockly.dialog.toast` and friends

[rachel-fenichel]

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Problem

In working on keyboard navigation we found that we needed a way to give feedback on certain actions, such as copy and paste. Toasts are a standard way to do this.

Like prompt and alert, this should have a basic implementation and be overrideable by the parent application, so that the style of the toast matches the rest of the page.

Request

• Create toast, toastImplementation and setToast in core/dialog.ts.
• Create a base implementation of toast.
• Add tests.

Alternatives considered

Implement the toast in Blockly core and only allow the parent application to change it through CSS.

Additional context

Toasts can have accessibility issues. More research is needed to see if there is a consensus within the accessibility community about how to do this well. The results may change our base implementation of toast and/or lead to published guideslines about the toast.

See RaspberryPiFoundation/blockly-keyboard-experimentation#230

[rachel-fenichel]

This will unblock RaspberryPiFoundation/blockly-keyboard-experimentation#124