Add Immutable object example

[anewman15]

Add the immutable object example used in the Mutable article here: https://developer.mozilla.org/en-US/docs/Glossary/Mutable. I think it's more appropriate here

Summary

Motivation

Supporting details

Related issues

Metadata

• [ ] Adds a new document
• [ ] Rewrites (or significantly expands) a document
• [ ] Fixes a typo, bug, or other error

[github-actions[bot]]

Preview URLs

• https://pr18728.content.dev.mdn.mozit.cloud/en-US/docs/Glossary/Immutable

Flaws

None! 🎉

External URLs

URL: /en-US/docs/Glossary/Immutable Title: Immutable on GitHub

No new external URLs

[sideshowbarker]

@anewman15 This is waiting on your response to https://github.com/mdn/content/pull/18728#pullrequestreview-1048629993