chore: replace deprecated mdx vscode extension

[jquinc30]

Changes description

Update recommendation (.vscode/extensions.json) for mdx vscode extension from silvenon.mdx to unifiedjs.vscode-mdx

Context

This extension is deprecated and point to a new one : unifiedjs.vscode-mdx

Checklist

• [x] Make sure you are requesting to pull a topic/feature/bugfix branch. Please, don't request directly from your main!
• [x] Check commits & PR names matches our requested structure. It must follow the https://www.conventionalcommits.org pattern.
• [x] Check your code additions will fail neither code linting checks.
• [x] I have reviewed the submitted code.
• [x] I have tested on related showcases.
• [x] If it includes design changes, please ask for a review design-system-core-team-design GitHub team.

Does this introduce a breaking change?

• No

Other information

[CLAassistant]

All committers have signed the CLA.