Add JS Question on Valid Variable Names

[khairalanam]

Summary of changes

This PR adds a question to the JavaScritp Quiz on valid variable names

Checklist

• [x] I have read and followed the contribution guidelines.
• [x] I have read through the Code of Conduct and agree to abide by the rules.
• [x] This PR is for one of the available issues and is not a PR for an issue already assigned to someone else.
• [x] My PR title has a short descriptive name so the maintainers can get an idea of what the PR is about.
• [x] I have provided a summary of my changes.