Port of @bethylamine's Soupcan extension to Safari

Notes:

Converted using

xcrun safari-web-extension-converter --app-name Soupcan --bundle-identifier net.acrossthecloud.Soupcan --project-location . --swift soupcan-extension/manifest-v3

Known issues:

• The conversion step gave a warning to say the open_in_tab option for the options UI is unsupported, but Safari settings > Extensions > Soupcan > Settings successfully brings up the options page.
• "The service_worker script failed to load due to an error." The service_worker script is set to background.js, but still seems to run.
• Safari settings > Extensions > Soupcan > PERMISSIONS > needed to be manaully set to twitter.com > Allow, on iOS, but worked with Ask on macOS.
• For development, you will probably want to switch off signing for all four targets (the apps and the corresponding extensions), unless you have a code signing certificate, and at least on macOS configure Safari to run unsigned extensions (I don't know about iOS)