Added dismissible alert plugin

This is the cleaned up version of https://github.com/wet-boew/wet-boew/pull/5932.
Please note the build will fail because of this issue: https://github.com/wet-boew/wet-boew/issues/5927
So identical visual alerts could in some cases include headings and other case not? How is that a good thing for the end user? That means the visual semantics will be out of sync with the programmatic/structural semantics which is precisely what the WCAG failure is trying to prevent. If they need to have different impacts on the document outline then they shouldn't look the same visually (and using alerts in both cases may not be the most appropriate solution).