Move common `ui-playground` components into a `shared` directory

In the soon-future, we'll want to extract the common bits of the
UI Playground app to the `frontend-shared` repository. This is step 1.

dev-server/ui-playground/components/patterns/MenuPatterns.js

-import {
-  PatternPage,
-  Pattern,
-  PatternExamples,
-  PatternExample,
-} from '../PatternPage';
-
-import Menu from '../../../../src/sidebar/components/Menu';
-import MenuItem from '../../../../src/sidebar/components/MenuItem';
-
-export default function MenuPatterns() {
-  return (
-    <PatternPage title="Menu">
-      <Pattern title="Menu">
-        <p>A simple Menu usage example</p>
-        <PatternExamples>
-          <PatternExample details="Menu">
-            <Menu title="Example Menu" label="Edit">
-              <MenuItem label="Zoom in" />
-              <MenuItem label="Zoom out" />
-              <MenuItem label="Undo" />
-              <MenuItem label="Redo" />
-            </Menu>
-          </PatternExample>
-        </PatternExamples>
-      </Pattern>
-    </PatternPage>
-  );
-}

dev-server/ui-playground/index.js

 import { registerIcons } from '@hypothesis/frontend-shared';
 import { render } from 'preact';
 
-import PlaygroundApp from './components/PlaygroundApp';
+import PlaygroundApp from './shared/components/PlaygroundApp';
 
 import sidebarIcons from '../../src/sidebar/icons';
 registerIcons(sidebarIcons);

dev-server/ui-playground/components/PlaygroundApp.js → dev-server/ui-playground/shared/components/PlaygroundApp.js

 import { SvgIcon } from '@hypothesis/frontend-shared';
 
-import MenuPatterns from './patterns/MenuPatterns';
 import SharedButtonPatterns from './patterns/SharedButtonPatterns';
 
 import { useRoute } from '../router';
@@ -20,11 +19,6 @@ const routes = [
     title: 'Home',
     component: HomeRoute,
   },
-  {
-    route: '/menu',
-    title: 'Menu',
-    component: MenuPatterns,
-  },
   {
     route: '/shared-buttons',
     title: 'Buttons (Shared)',

dev-server/ui-playground/components/patterns/SharedButtonPatterns.js → dev-server/ui-playground/shared/components/patterns/SharedButtonPatterns.js

@@ -9,7 +9,7 @@ import {
   IconButton,
   LabeledButton,
   LinkButton,
-} from '../../../../src/shared/components/buttons';
+} from '../../../../../src/shared/components/buttons';
 
 export default function SharedButtonPatterns() {
   return (