misc: make copyUntypedFiles work for watch mode

[Josh-Cena]

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

The last missing bit of DX. copyUntypedFiles.mjs now work for watch mode as well, with a simple chokidar watcher.

Test Plan

We can now hot-reload CSS as well! For example, here let's pretend we are editing Details/styles.module.css for theme-common (which used to not work):

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	16d705d
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/6284d304586b800009f3f096
😎 Deploy Preview	https://deploy-preview-7445--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 settings.

[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 89	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 82	🟢 99	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: 0 B

Total Size: 796 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.3 kB
website/build/assets/css/styles.********.css	106 kB
website/build/assets/js/main.********.js	599 kB
website/build/index.html	38.9 kB

_{compressed-size-action}

[slorber]

great 👍 thanks