Introduce new TreeView UI component

Key highlights:

- Written from scratch to cater specifically to privacy.sexy's
  needs and requirements.
- The visual look mimics the previous component with minimal changes,
  but its internal code is completely rewritten.
- Lays groundwork for future functionalities like the "expand all"
  button a flat view mode as discussed in #158.
- Facilitates the transition to Vue 3 by omitting the Vue 2.0 dependent
  `liquour-tree` as part of #230.

Improvements and features:

- Caching for quicker node queries.
- Gradual rendering of nodes that introduces a noticable boost in
  performance, particularly during search/filtering.
  - `TreeView` solely governs the check states of branch nodes.

Changes:

- Keyboard interactions now alter the background color to highlight the
  focused item. Previously, it was changing the color of the text.
- Better state management with clear separation of concerns:
  - `TreeView` exclusively manages indeterminate states.
  - `TreeView` solely governs the check states of branch nodes.
  - Introduce transaction pattern to update state in batches to minimize
    amount of events handled.
- Improve keyboard focus, style background instead of foreground. Use
  hover/touch color on keyboard focus.
- `SelectableTree` has been removed. Instead, `TreeView` is now directly
  integrated with `ScriptsTree`.
- `ScriptsTree` has been refactored to incorporate hooks for clearer
  code and separation of duties.
- Adopt Vue-idiomatic bindings instead of keeping a reference of the
  tree component.
- Simplify and change filter event management.
- Abandon global styles in favor of class-scoped styles.
- Use global mixins with descriptive names to clarify indended
  functionality.

tests/unit/shared/Stubs/UserSelectionStub.ts

@@ -3,19 +3,23 @@ import { SelectedScript } from '@/application/Context/State/Selection/SelectedSc
 import { IScript } from '@/domain/IScript';
 import { IEventSource } from '@/infrastructure/Events/IEventSource';
 import { EventSource } from '@/infrastructure/Events/EventSource';
+import { StubWithObservableMethodCalls } from './StubWithObservableMethodCalls';
 
-export class UserSelectionStub implements IUserSelection {
+export class UserSelectionStub
+  extends StubWithObservableMethodCalls<IUserSelection>
+  implements IUserSelection {
   public readonly changed: IEventSource<readonly SelectedScript[]> = new EventSource<
   readonly SelectedScript[]>();
 
   public selectedScripts: readonly SelectedScript[] = [];
 
   constructor(private readonly allScripts: readonly IScript[]) {
-
+    super();
   }
 
-  public withSelectedScripts(selectedScripts: readonly SelectedScript[]) {
+  public withSelectedScripts(selectedScripts: readonly SelectedScript[]): this {
     this.selectedScripts = selectedScripts;
+    return this;
   }
 
   public areAllSelected(): boolean {
@@ -34,16 +38,22 @@ export class UserSelectionStub implements IUserSelection {
     throw new Error('Method not implemented.');
   }
 
-  public addSelectedScript(): void {
-    throw new Error('Method not implemented.');
+  public addSelectedScript(scriptId: string, revert: boolean): void {
+    this.registerMethodCall({
+      methodName: 'addSelectedScript',
+      args: [scriptId, revert],
+    });
   }
 
   public addOrUpdateSelectedScript(): void {
     throw new Error('Method not implemented.');
   }
 
-  public removeSelectedScript(): void {
-    throw new Error('Method not implemented.');
+  public removeSelectedScript(scriptId: string): void {
+    this.registerMethodCall({
+      methodName: 'removeSelectedScript',
+      args: [scriptId],
+    });
   }
 
   public selectOnly(scripts: ReadonlyArray<IScript>): void {