* Reverting changes in https://github.com/woocommerce/woocommerce/pull/30380, preparatory to merging the My Subscriptions and Marketplace pages back into one Extensions page.
* Moved category nav to a separate file.
* Added main nav tabs.
* Added category nav, styled the mobile version.
* Experiments with doing category tabs as a list.
* Styling category menu as a list of links in rectangular pills on desktop.
* Adjusting margins of product list in in-app marketplace.
* Removed `submenu_file` filter which called deleted `update_menu_highlight` method.
* Restored styling for update count bubble on My Subscriptions tab.
* Styling update bubble on My Subscriptions tab.
* Addressing design feedback. Restyled category pills to make them more obvious. Added `Browse categories` title before category menu on desktop. Changed `.addon-product-group-title` to an h2.
* Addressing feedback from https://github.com/woocommerce/woocommerce/pull/31085#pullrequestreview-806319880.
- Removing addition of `admin_title` filter calling deleted `update_my_subscriptions_title` method.
- Prepending `__DIR__` constant before path in `require` call.
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