Fixing Singleton typing to correctly return child class (#1914)

- Add distinction between `getInstance` and `getInstanceOrCreate` since
  it is not always possible to create an instance (since you might not
  know the correct arguments)

- Remove all the `export const *Store = *Store.getInstance<*Store>();`
  calls as it defeats the purpose of `Singleton`. Plus with the typing
  changes the appropriate `*Store.getInstance()` is "short enough".

- Special case the two extension export facades to not need to use
  `getInstanceOrCreate`. Plus since they are just facades it is always
  possible to create them.

- Move some other types to be also `Singleton`'s: ExtensionLoader,
  ExtensionDiscovery, ThemeStore, LocalizationStore, ...

- Fixed dev-run always using the same port with electron inspect

- Update Store documentation with new recommendations about creating
  instances of singletons

- Fix all unit tests to create their dependent singletons

Signed-off-by: Sebastian Malton <sebastian@malton.name>

src/renderer/components/layout/__test__/main-layout-header.test.tsx

@@ -6,6 +6,7 @@ import "@testing-library/jest-dom/extend-expect";
 
 import { MainLayoutHeader } from "../main-layout-header";
 import { Cluster } from "../../../../main/cluster";
+import { ClusterStore } from "../../../../common/cluster-store";
 
 const cluster: Cluster = new Cluster({
   id: "foo",
@@ -14,6 +15,14 @@ const cluster: Cluster = new Cluster({
 });
 
 describe("<MainLayoutHeader />", () => {
+  beforeEach(() => {
+    ClusterStore.getInstanceOrCreate();
+  });
+
+  afterEach(() => {
+    ClusterStore.resetInstance();
+  });
+
   it("renders w/o errors", () => {
     const { container } = render(<MainLayoutHeader cluster={cluster} />);