Sorry, there isn't a way to add additional search engines to the Search engine: setting in Safari's Search preferences.
Salvodelli
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I'm not able to reproduce this problem when using browser.storage.onChanged from a background page, it works as expected for me there. I copied your sample code verbatim.
Are you seeing this failing when registering the event handler from some other context (e.g. from an injected content script in other extension content like a popup)?
You need to have the beta of Safari 14 installed to use safari-web-extension-converter. You can download it from here - https://developer.apple.com/download/more/?=Safari.