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
Please nest this and the other classes. For instance you would do:

details {
	&.alert {
	}

	&.alert-success {
	}

	...
}
```

The same should be done for section.