[Bug][a11y]: Toast should have API for overriding the icon's alt text

[majornista]

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Code of conduct

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Impacted component(s)

Toast

Expected behavior

It should be possible to override the string used as the label for the sp-icon element in the Shadow DOM, both for i18n purposes and for clarity, because sometimes an "Warning" is not an "Error."

This relates to https://github.com/adobe/spectrum-web-components/issues/1975

Actual behavior

The alt text for the icon displayed in the "negative", "positive", and "info" Toast variants is hardcoded and cannot be overridden as "Error", "Success", and "Information" respectively.

Screenshots

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What browsers are you seeing the problem in?

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How can we reproduce this issue?

1. Go to '...'
2. Click on '....'
3. Scroll to '....'
4. Check console
5. See error

Sample code that illustrates the problem

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Logs taken while reproducing problem

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