Use wp.i18n for translatable strings in wp-admin/js/nav-menu.js

[ocean90]

We should replace the use of wp_localize_script() with the proper JavaScript i18n functions introduced in WordPress 5.0, see #20491 and #45103. This ticket is for wp-admin/js/nav-menu.js.

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[ocean90]

In 48394:

I18N: Use wp.i18n for translatable strings in wp-admin/js/nav-menu.js.

This removes the usage of wp_localize_script() for passing translations to the script and instead adds the translatable strings in the script directly through the use of wp.i18n and its utilities.

Props swissspidy, ocean90.
See #20491.
Fixes #50603.

[justinahinon]

Related dev note: ​https://make.wordpress.org/core/2020/08/05/more-support-for-javascript-i18n-in-wordpress-5-5/