Update DemoJS docs tab processing to match new raw-loader import format

[chandlerprall]

Summary

raw-loader was upgraded in #4022 which changed the result of requireing to an ESModule format instead of returning the default export. This PR accesses updates the guide component to add .default when accessing the string value.

A more correct change would be to update every instance of require('!!raw-loader! to import from '!!raw-loader! but that

1. introduces an eslint warning
2. requires an automated change to every single example file

These should be solvable, but I'm opting for a quick fix to get the DemoJS tabs working again before going down the more complex path.

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes
- [ ] Checked in mobile
- [ ] Checked in Chrome, Safari, Edge, and Firefox
- [ ] Props have proper autodocs
- [ ] Added documentation

• Checked Code Sandbox works for the any docs examples
  - [ ] Added or updated jest tests
  - [ ] Checked for breaking changes and labeled appropriately
  - [ ] Checked for accessibility including keyboard-only and screenreader modes
  - [ ] A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4027/

[cchaos]

It's working! 🎉