Manual task reordering feature (#6)

This PR contains the work done to add support for manual todo task reordering in the *Task list* view

Reviewed-on: rock-n-code/todos-app#6
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

App/Sources/Models/Todo.swift

@@ -16,6 +16,9 @@ final class Todo {
     /// An optional note providing additional details about the todo task.
     var note: String?
 
+    /// The display order of the todo task.
+    var order: Int
+
     /// The title of the todo task.
     var title: String
 
@@ -27,15 +30,18 @@ final class Todo {
     ///   - note: An optional note providing additional details.
     ///   - due: The date by which the todo should be completed.
     ///   - completed: The date when the todo was completed, if applicable.
+    ///   - order: The display order of the todo task.
     init(
         title: String,
         note: String? = nil,
         due: Date,
-        completed: Date? = nil
+        completed: Date? = nil,
+        order: Int = 0
     ) {
         self.completed = completed
         self.due = due
         self.note = note
+        self.order = order
         self.title = title
     }