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/locale-date/locale-date.tsx

@@ -1,7 +1,7 @@
 import React from "react";
 import { observer } from "mobx-react";
 import moment from "moment-timezone";
-import { userStore } from "../../../common/user-store";
+import { UserStore } from "../../../common/user-store";
 
 interface Props {
   date: string
@@ -10,7 +10,7 @@ interface Props {
 @observer
 export class LocaleDate extends React.Component<Props> {
   render() {
-    const { preferences } = userStore;
+    const { preferences } = UserStore.getInstance();
     const { date } = this.props;
 
     return <>{moment.tz(date, preferences.localeTimezone).format()}</>;