Add an accordion widget

[rodrigogiraoserrao]

(This issue is reserved for the EuroPython 2023 sprint. If you are not participating in the sprint, please refrain from working on this issue. Thanks!)

This depends on #2918.

This issue concerns itself with adding a new accordion widget to Textual.
An accordion is a collection of mutually exclusive collapsible containers.
An accordion can contain any number of collapsible containers, but only one may be expanded at a time.

Roughly speaking, an accordion widget will be to the collapsible containers what the RadioSet widget is to the RadioButton widgets.
In fact, take a look at _radio_set.py for some inspiration with regards to how you can orchestrate the collapsible containers inside the accordion.

Take a look at #2333 to see what files need to be changed when you implement a new widget.
Then, don't be like me and also do the final step shown in #2399 to add your new widget to the widget gallery.

Good luck and have fun!

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