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

[yosukekato165]

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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

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

[slorber]

LGTM thanks 👍