Fix type definition of TheiaAppFactory (#13799)

Ensure that the `TheiaAppFactory` type does  match the constructor function of TheiaApp.
`InitialWorkspace` is required in the TheiaApp constructor butwas optional in `TheiaAppFactory`
This ensures that (sub)classes of TheiaApp can properly be used as app factory without type casting

Contributed on behalf of STMicroelectronics

examples/playwright/src/theia-app-loader.ts

@@ -22,7 +22,7 @@ import { TheiaWorkspace } from './theia-workspace';
 import { OSUtil } from './util';
 
 export interface TheiaAppFactory<T extends TheiaApp> {
-    new(page: Page, initialWorkspace?: TheiaWorkspace, isElectron?: boolean): T;
+    new(page: Page, initialWorkspace: TheiaWorkspace, isElectron?: boolean): T;
 }
 
 // TODO this is just a sketch, we need a proper way to configure tests and pass this configuration to the `TheiaAppLoader`: