Add a skeleton/placeholder component

[edward-ly]

A similar component has already been implemented in the Talk app, but it would be nice if this component could be available in the library so that other apps could use it too.

Perhaps for a generic component, the chat message skeleton above can be broken down into individual shapes (e.g. like this) so that it can also be used in other contexts outside of chat.

Related: nextcloud/assistant#214

[ShGKme]

I'd consider making it CSS first. This allows using placeholders during app loading:

• Example: fix(AdminSettings): remove PHP template nextcloud/spreed#12839

[ShGKme]

Or SVG