Simplify creating store modules with static initial state

Allow the `initialState` argument to `createStoreModule` to be a value
to simplify usage for modules where the initial state is a static value
(ie. has no dependency on data in `settings`).

src/sidebar/store/create-store.js

@@ -197,7 +197,7 @@ export function createStore(modules, initArgs = [], middleware = []) {
  * @template Actions
  * @template {SelectorMap<State>} Selectors
  * @template RootSelectors
- * @param {(...args: any[]) => State} initialState
+ * @param {State | ((...args: any[]) => State)} initialState
  * @param {object} config
  *   @param {string} config.namespace -
  *     The key under which this module's state will live in the store's root state
@@ -211,6 +211,12 @@ export function createStoreModule(initialState, config) {
   // The `initialState` argument is separate to `config` as this allows
   // TypeScript to infer the `State` type in the `config` argument at the
   // `createStoreModule` call site.
+
+  if (!(initialState instanceof Function)) {
+    const state = initialState;
+    initialState = () => state;
+  }
+
   return {
     initialState,
     ...config,

src/sidebar/store/test/create-store-test.js

@@ -173,6 +173,18 @@ describe('createStore', () => {
     assert.equal(store.getState().b.count, 0);
   });
 
+  it('supports modules with static initial state', () => {
+    const initialState = { value: 42 };
+    const module = createStoreModule(initialState, {
+      namespace: 'test',
+      actionCreators: {},
+      reducers: {},
+      selectors: {},
+    });
+    const store = createStore([module]);
+    assert.equal(store.getState().test.value, 42);
+  });
+
   if (process.env.NODE_ENV !== 'production') {
     it('freezes store state in development builds', () => {
       const store = counterStore();