[Kibana Overview] Shift SCSS import and lazy-load main component (#20…

…4661)

## Summary

Part of elastic/kibana-team#1082

The purpose of this PR is to reduce the page load bundle size of the
`kibanaOverview` plugin.

Converts the main component of the `kibanaOverview` plugin to be lazy
loaded, and shifts the import of the main SCSS file to be imported from
the lazily-loaded component. This PR does not include any changes that
would be noticeable by end-users. It changes the internals to use a
different technology for styling components.

## References
1. https://emotion.sh/docs/globals
2. https://emotion.sh/docs/best-practices
3.
elastic/eui#6828 (comment)

src/plugins/kibana_overview/public/components/app.tsx

@@ -13,11 +13,11 @@ import { I18nProvider } from '@kbn/i18n-react';
 import type { NavigationPublicPluginStart } from '@kbn/navigation-plugin/public';
 import type { FetchResult } from '@kbn/newsfeed-plugin/public';
 import { Route, Routes } from '@kbn/shared-ux-router';
+import { withSuspense } from '@kbn/shared-ux-utility';
 import React, { useEffect, useState } from 'react';
 import { HashRouter as Router } from 'react-router-dom';
 import useObservable from 'react-use/lib/useObservable';
 import type { Observable } from 'rxjs';
-import { Overview } from './overview';
 
 interface KibanaOverviewAppDeps {
   basename: string;
@@ -48,6 +48,14 @@ export const KibanaOverviewApp = ({
     }
   }, [newsfeed$]);
 
+  const Overview = withSuspense(
+    React.lazy(() =>
+      import('./overview').then(({ Overview: OverviewComponent }) => {
+        return { default: OverviewComponent };
+      })
+    )
+  );
+
   return (
     <Router basename={basename}>
       <I18nProvider>

src/plugins/kibana_overview/public/components/overview/overview.tsx

@@ -7,6 +7,8 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
+import './overview.scss';
+
 import { snakeCase } from 'lodash';
 import React, { FC, useState, useEffect } from 'react';
 import useObservable from 'react-use/lib/useObservable';

src/plugins/kibana_overview/public/index.ts

@@ -7,8 +7,6 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
-import './index.scss';
-
 import { KibanaOverviewPlugin } from './plugin';
 
 // This exports static code and TypeScript types,