Foundations: Replace links to foundations JS exercises

[mao-sz]

Because

Exercises repo restructured in TheOdinProject/javascript-exercises#597

This PR

• Replaces exercises links with foundations/ dir version

Issue

N/A

Must only be merged alongside TheOdinProject/javascript-exercises#597

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• 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
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide