Move view model code to shared components package (#31024)

* Remove vm related code from element-web/src

* Add and export view model code from package

* Update imports

* Rewrite vm tests using vitest

* Add github action to run vm tests

* Fix lint errors

* Mvoe tests over to jest

* Try fixing code coverage

* Second attempt at fixing code coverage

packages/shared-components/src/viewmodel/ViewModelSubscriptions.ts

@@ -0,0 +1,34 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+/**
+ * Utility class for view models to manage subscriptions to their updates
+ */
+export class ViewModelSubscriptions {
+    private listeners = new Set<() => void>();
+
+    /**
+     * Subscribe to changes in the view model.
+     * @param listener Will be called whenever the snapshot changes.
+     * @returns A function to unsubscribe from the view model updates.
+     */
+    public add = (listener: () => void): (() => void) => {
+        this.listeners.add(listener);
+        return () => {
+            this.listeners.delete(listener);
+        };
+    };
+
+    /**
+     * Emit an update to all subscribed listeners.
+     */
+    public emit = (): void => {
+        for (const listener of this.listeners) {
+            listener();
+        }
+    };
+}