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
The reason for this change was lost in the commit messages I squashed, but it's to normalize the style of the polyfill versus a native implementation like Chrome's. Otherwise, the pointer is too big.