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
<blockquote>If it is important to single out alerts so much visually</blockquote>
Sometimes, you do that because you want to make the alert be easier to ignore. For instance, a notice which is not necessary to always read. These are the kinds of contextual notices where they shouldn't be in the outline, screen reader or not.

<blockquote>rather than showing different approaches to the same alert <strong>without any explanation or justification for why they are different</strong></blockquote>

I said: “and explain the use case”

Of course, we won't be just adding a different way of coding. We'd also be documenting why it's made that way.

Also, implementers are not people who have a hard time using Word. For instance, the people on my team are constantly puzzled by the lack of options and undocumented arbitrary markup decisions just like the one you're trying to establish.

On the contrary, it would be much better if there were options in usage, because really, the elements dictate that there are, just as those puzzled implementers know, which is why they're asking so many questions in the first place, and that these options would be documented and reasoned about in the context of WET.