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/ViewModel.ts

@@ -0,0 +1,23 @@
+/*
+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.
+*/
+
+/**
+ * The interface for a generic View Model passed to the shared components.
+ * The snapshot is of type T which is a type specifying a snapshot for the view in question.
+ */
+export interface ViewModel<T> {
+    /**
+     * The current snapshot of the view model.
+     */
+    getSnapshot: () => T;
+
+    /**
+     * Subscribes to changes in the view model.
+     * The listener will be called whenever the snapshot changes.
+     */
+    subscribe: (listener: () => void) => () => void;
+}