fix(mdx-loader): ensure heading anchor slugs respect GitHub emoji behavior

[yosukekato165]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Fix #9194

Test Plan

Test links

Deploy preview: https://deploy-preview-9202--docusaurus-2.netlify.app/tests/blog/2023/08/05/

Related issues/PRs

#9194

[facebook-github-bot]

Hi @yosukekato165!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!

[netlify]

✅ [V2]

Built without sensitive environment variables

Name	Link
🔨 Latest commit	e69b863
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/64ce2611483b3d00082ff80b
😎 Deploy Preview	https://deploy-preview-9202--docusaurus-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟢 92	🟢 97	🟢 92	🟢 100	🟠 89	Report
/docs/installation	🟠 75	🟢 100	🟢 92	🟢 100	🟠 89	Report

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[argos-ci]

The latest updates on your projects. Learn more about Argos notifications ↗︎

Build	Status	Details	Updated (UTC)
default (Inspect)	🧿 Changes detected (Review)	89 changes	Aug 10, 2023, 3:52 PM

[slorber]

LGTM thanks 👍