Update VSCode Extensions links in readme

[rmccar]

What is the context of this PR?

Fixes: #3185

Updates the VSCode Extension links in the readme and adds "axe Accessibility Linter" and "SonarLint". Also updates the title because these are now called Extensions.

How to review this PR

• Check links work, make sense and go to the correct VSCode Extensions
• Ideally these should be installed in all our IDEs

Checklist

This needs to be completed by the person raising the PR.

• [x] I have selected the correct Assignee
• [x] I have linked the correct Issue

[netlify[bot]]

✅ Deploy Preview for ons-design-system-preview ready!

Name	Link
🔨 Latest commit	16faa367bd86a30f15f72c3a5b60775eef4a2d84
🔍 Latest deploy log	https://app.netlify.com/sites/ons-design-system-preview/deploys/6645d8a4eae8720007b21ab8
😎 Deploy Preview	https://deploy-preview-3186--ons-design-system-preview.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.