Fix Storybook type annotations (#1091)

## What are you changing?

- Updates type annotations in Source Storybook stories.

## Why?

- All of the stories have a number of type errors. Whilst these (mostly)
don't prevent the stories from running or rendering in Chromatic, the
errors create a lot of visual noise in the code editor and can cause
confusion when working on them.

Author: jamesmockett

libs/@guardian/source-react-components-development-kitchen/.storybook/preview/FocusManagerDecorator.tsx

@@ -1,7 +1,8 @@
 import { useEffect } from 'react';
 import { FocusStyleManager } from '@guardian/source-foundations';
+import { Decorator } from '@storybook/react';
 
-export const FocusManagerDecorator = (storyFn) => {
+export const FocusManagerDecorator: Decorator = (storyFn) => {
 	useEffect(() => {
 		FocusStyleManager.onlyShowFocusOnTabs();
 	});

libs/@guardian/source-react-components-development-kitchen/.storybook/preview/ThemeProviderDecorator.tsx

@@ -1,6 +1,7 @@
 import { ThemeProvider } from '@emotion/react';
+import { Decorator } from '@storybook/react';
 
-export const ThemeProviderDecorator = (storyFn, context) => {
+export const ThemeProviderDecorator: Decorator = (storyFn, context) => {
 	const theme = context.parameters.theme;
 	return theme ? (
 		<ThemeProvider theme={theme}>{storyFn()}</ThemeProvider>