Assembler - make sure only one snackbar of same type displays at once

[miksansegundo]

What

Stacking multiple notices obscures the interface in the assembler.

Why

For better visibility of the canvas. Following the same behavior as in the editor. See WordPress/gutenberg#50625

How

Let's show only one notice of the same type (add/replace/remove) at a time.

[fushar]

This is shipped!