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
Looks good to me. Did you figure out how to make the pointer size to the heading? There are cases where an h2 or h4 could be used for the alert (such as the top-level section and sub-section alerts in the style guide). That was something that people were struggling with previously when trying to use headings in summary elements so whatever is used here could be used to help with other users.