Deprecate externalCss

[nvh95]

Related issue

• Depreacate externalCss option in jestPreviewConfigure #124

Features

• Deprecate externalCss
• Add a warning when users try to use externalCss
• Add a blog post to announce about the deprecation
• Update docs

Chores

• Update examples not to use externalCss

TLDR

// src/setupTests.js
-jestPreviewConfigure({
-  externalCss: ['src/global.css', 'src/assets/_scss/global-style.scss'],
-});
+import './global.css';
+import './assets/_scss/global-style.scss';

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