chore: bump @storybook/addon-essentials from 6.3.12 to 6.4.18 in /site (#173)

Bumps [@storybook/addon-essentials](https://github.com/storybookjs/storybook/tree/HEAD/addons/essentials) from 6.3.12 to 6.4.18.
- [Release notes](https://github.com/storybookjs/storybook/releases)
- [Changelog](https://github.com/storybookjs/storybook/blob/next/CHANGELOG.md)
- [Commits](https://github.com/storybookjs/storybook/commits/v6.4.18/addons/essentials)

---
updated-dependencies:
- dependency-name: "@storybook/addon-essentials"
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

site/package.json

@@ -23,7 +23,7 @@
     "@material-ui/lab": "4.0.0-alpha.42",
     "@react-theming/storybook-addon": "1.1.5",
     "@storybook/addon-actions": "6.3.12",
-    "@storybook/addon-essentials": "6.3.12",
+    "@storybook/addon-essentials": "6.4.18",
     "@storybook/addon-links": "6.3.12",
     "@storybook/react": "6.4.18",
     "@testing-library/react": "12.1.2",