Moves the 'loading' and 'loaded' i18n strings for the `interactivity-router` to the script module data via the `script_module_data_@wordpress/interactivity-router` filter.
Key changes:
- Add the `filter_script_module_interactivity_router_data()` method, hooked into the `script_module_data_@wordpress/interactivity-router` filter, to set the `i18n` data with the 'loading' and 'loaded' messages.
- Rename the `print_router_loading_and_screen_reader_markup()` method to `print_router_markup()` and remove the screen reader markup from it because it's no longer needed.
- Remove the `loading` and `loaded` strings from the `core/router` store state because they're no longer needed.
- Initialize the `core/router` store with a minimal navigation object to prevent errors in the interactivity-router script module when the store is not properly initialized.
- Update corresponding unit tests to reflect these changes.
This change ensures that the `interactivity-router` i18n messages are localized in a single place and removes the need to initialize them in the `core/router` store state.
Props jonsurrell, swissspidy, czapla.
See #60647.
Built from https://develop.svn.wordpress.org/trunk@59097
git-svn-id: http://core.svn.wordpress.org/trunk@58493 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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