feat(web): get list of store names from stores defined in core
This implements `CoreKeyboardInterface.loadSerializedOptions` by getting the list of all variable store names from Core and then trying to load these.
Also change `VariableStoreSerializer.loadStore` signature so that it can return `undefined`. This allows to return only those options from `loadSerializedOptions` that have a non-default value, i.e. were serialized before.
Follow-up-of: #15470
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