Add Option open and close in accordion

At the moment there is a global option at `/admin/config/content/ckeditor-accordion`. But having an option for the individual accordions requires some changes.

I took a stab at updating the plugin to have the option, but haven't finished implementing the feature. The issue fork has a working UI.

I'm newish to ckeditor plugins so still learning the options for implementing widgets like the form input and balloon selection.

I got a lot of it from https://ckeditor.com/docs/ckeditor5/latest/tutorials/abbreviation-plugin....

Some things to consider.

For a single accordion, its straightforward, just open or closed by default. But how to account for the global setting? Should we default to the global and treat this as an override? If the global is switched, it wouldn't change existing accordions that have this setting changed, unless the options are open, closed, default.

For multiple accordion rows, there is a global option to open one at a time. Maybe that should be an override per accordion as well. If it's set, the UI should let you pick the one to open and not allow selecting multiple. If it's not set, the open/closed setting could be set per row.

So I think there should be some settings per accordion - open/close all or open one at a time. Then per accordion row, conditionally allow setting open/closed.