Add an additional resource to javascript classes lesson

[GabGaitanidis]

JavaScript Classes: additional resource

Because

Personally, I believe that the best way to learn a concept is by doing exercises. This website offers a collection of javascript classes exercises. This will help beginners have a better understanding on js classes as they will aply what they have learned.

This PR

• Adds an additional resource

Issue

Closes #XXXXX

Additional Information

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project curriculum contributing guide
• [x] The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] If this PR addresses an open issue, it is linked in the Issue section
• [x] If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• [x] If any lesson files are included in this PR, they follow the Layout Style Guide