Initial implementation of AccountSettings. (#6275)

* Make the UserPreference macro also generate an accessor for the key name.

* Initial implementation of AccountSettings.

- Add an AccountUserDefaults storage with a userID prefix on the key.
- Migrate the 2 values reset by AppSettings.resetSessionSpecificSettings to AccountSettings.

* Rename UserSettings.volatile() to UserSettings.mock()

Given it now has a userID parameter with a default value..

Components/BuildExtensions/Sources/MacrosImplementation/UserPreferenceMacro.swift

@@ -45,6 +45,10 @@ enum UserPreferenceMacro {
             "_\(raw: name)VolatileValue"
         }
         
+        var keyPropertyName: TokenSyntax {
+            "\(raw: name)Key"
+        }
+        
         var resetMethodName: TokenSyntax {
             "reset\(raw: name.prefix(1).uppercased() + name.dropFirst())"
         }
@@ -157,6 +161,14 @@ extension UserPreferenceMacro: PeerMacro {
         }
         """)
         
+        if !preference.isVolatile {
+            peers.append("""
+            var \(preference.keyPropertyName): UserPreferenceKey {
+                .init(\(preference.key))
+            }
+            """)
+        }
+        
         peers.append(resetMethod(for: preference))
         
         return peers