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/+whats-new/whats-new.tsx

@@ -3,7 +3,7 @@ import fs from "fs";
 import path from "path";
 import React from "react";
 import { observer } from "mobx-react";
-import { userStore } from "../../../common/user-store";
+import { UserStore } from "../../../common/user-store";
 import { navigate } from "../../navigation";
 import { Button } from "../button";
 import marked from "marked";
@@ -14,7 +14,7 @@ export class WhatsNew extends React.Component {
 
   ok = () => {
     navigate("/");
-    userStore.saveLastSeenAppVersion();
+    UserStore.getInstance().saveLastSeenAppVersion();
   };
 
   render() {