refactor: switch to TypeScript

Aside from introducing a bunch of type annotations and other code
adjustments, also add explicit type checking where necessary.

Inline #associateItem into the constructor in PrefsBoxOrderItemRow as
the method sets this.item and:
> Note that the field needs to be initialized in the constructor itself.
> TypeScript does not analyze methods you invoke from the constructor to
> detect initializations, because a derived class might override those
> methods and fail to initialize the members.
https://www.typescriptlang.org/docs/handbook/2/classes.html

Explicitly ensure we actually have a Gdk.Drag in #setupDNDScroll in
PrefsPage and explicitly only scroll when a DND operation is properly
set up. Even tho previously not having a Gdk.Drag in #setupDNDScroll
would probably just error out the callback and probably be just fine
then, handling this explicitly is at least nicer.

Also see the guide on using TypeScript for GNOME Shell Extensions, which
was followed for this work to some degree:
https://gjs.guide/extensions/development/typescript.html

src/extensionModules/BoxOrderManager.ts

@@ -215,8 +215,13 @@ export default class BoxOrderManager extends GObject.Object {
                 roles = this.#taskUpUltraLiteItemRoles;
             }
 
-            // Create a new resolved box order item for each role and add it to
+            // If there are no roles associated, continue.
-            // the resolved box order.
+            if (roles.length === 0) {
+                continue;
+            }
+
+            // Otherwise create a new resolved box order item for each role and
+            // add it to the resolved box order.
             for (const role of roles) {
                 const newResolvedBoxOrderItem = JSON.parse(JSON.stringify(resolvedBoxOrderItem));
                 newResolvedBoxOrderItem.role = role;
@@ -254,15 +259,20 @@ export default class BoxOrderManager extends GObject.Object {
         // Get a resolved box order.
         let resolvedBoxOrder = this.#getResolvedBoxOrder(box);
 
+        // ToDo: simplify.
         // Get the indicator containers (of the items) currently present in the
         // GNOME Shell top bar.
         // They should be St.Bins (see link), so ensure that using a filter.
         // https://gitlab.gnome.org/GNOME/gnome-shell/-/blob/48.2/js/ui/panelMenu.js?ref_type=tags#L21
-        const indicatorContainers = new Set([
+        const indicatorContainers = [
             (Main.panel as CustomPanel)._leftBox.get_children(),
             (Main.panel as CustomPanel)._centerBox.get_children(),
             (Main.panel as CustomPanel)._rightBox.get_children(),
-        ].flat().filter(ic => ic instanceof St.Bin));
+        ].flat().filter(ic => ic instanceof St.Bin);
+
+        // Create an indicator containers set from the indicator containers for
+        // fast easy access.
+        const indicatorContainerSet = new Set(indicatorContainers);
 
         // Go through the resolved box order and only add items to the valid box
         // order, where their indicator is currently present in the GNOME Shell
@@ -275,7 +285,7 @@ export default class BoxOrderManager extends GObject.Object {
                 continue;
             }
 
-            if (indicatorContainers.has(associatedIndicatorContainer)) {
+            if (indicatorContainerSet.has(associatedIndicatorContainer)) {
                 validBoxOrder.push(item);
             }
         }

src/prefsModules/PrefsPage.ts

@@ -28,9 +28,9 @@ export default class PrefsPage extends Adw.PreferencesPage {
     }
 
     _dndEnded?: boolean;
-    declare _left_box_order_list_box: PrefsBoxOrderListBox;
+    _left_box_order_list_box!: PrefsBoxOrderListBox;
-    declare _center_box_order_list_box: PrefsBoxOrderListBox;
+    _center_box_order_list_box!: PrefsBoxOrderListBox;
-    declare _right_box_order_list_box: PrefsBoxOrderListBox;
+    _right_box_order_list_box!: PrefsBoxOrderListBox;
 
     constructor(params = {}) {
         super(params);