feat: add Stack effect

[yaodingyd]

Fix #16

Several major refactor/implementation details:

1. Instead of one instances array, now there's three arrays, each for one position. instances now is an object with three arrays properties, position being the key.
2. Adding a layer of container to hold snackbar--text and snackbar--button. This is needed for keeping the expanded status when hovering on snackbars. Previously, moving through expanded snackbars will go back to stack status because of the margin.
3. Rename the function and event to be animation related because hide is an animation. (This is a bug somewhat related to this change.)
4. Remove instance from array when destroyed. It should be from the beginning and it also effects the transition index if not cleared.

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Referenced issues

This pull request has been submitted to:

• #16: Stack effect

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[egoist]

If you move your mouse over the hidden snackbars it will expand 😅

[yaodingyd]

I don't think you last refactor (which resembles my initial tries) is correct. When we don't clear the setTimeout for setting elements hidden, if user clicks too fast and one snackbar is supposed to be visible, it's possible that last setTimeout will overwrite it's visibility and make it hidden again, like this:

[egoist]

🎉 This PR is included in version 1.5.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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