Add keyboard shortcut to match other MacOS Chat Apps (#4719)

MacOS Messages and WhatsApp (amongst other messaging apps) use this Cmd + Shift + [/] to switch between convos.  Would be great to have that in Signal as well.

ts/background.ts

@@ -1146,6 +1146,7 @@ type WhatIsThis = import('./window.d').WhatIsThis;
       // up/previous
       if (
         (!isSearching && optionOrAlt && !shiftKey && key === 'ArrowUp') ||
+        (!isSearching && commandOrCtrl && shiftKey && key === '[') ||
         (!isSearching && ctrlKey && shiftKey && key === 'Tab')
       ) {
         const unreadOnly = false;
@@ -1165,6 +1166,7 @@ type WhatIsThis = import('./window.d').WhatIsThis;
       // down/next
       if (
         (!isSearching && optionOrAlt && !shiftKey && key === 'ArrowDown') ||
+        (!isSearching && commandOrCtrl && shiftKey && key === ']') ||
         (!isSearching && ctrlKey && key === 'Tab')
       ) {
         const unreadOnly = false;