[safari] things to fix

[kouks]

• safari.self x safari.application
• content script being reinjected over and over again
• safari storage returns null, not undefined when key is not found
• event.target.tab instead of page in messaging
• bind Messaging as a singleton
• check if respond promise exists (both content scripts are picking up responses from the background script)
• new tab is always active
• add check for window.top === window before loading the script
• make cli bundle the exension to {name}.safariextension
• solve relative URLs in the extension as safari appens a random string... 🥇