Fix Typing for TableCell backgroundColor prop

[natalynyu]

This PR addresses this error in the Course Planning app:

Individual areas cannot be accessed without casting using ColorRange, which was a broader type than required. Instead of ColorRange as a type, I set the type for area to { [key: string]: string };

Types of changes

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have run eslint on the code
• [x] I have added JSDoc for all of my code (where applicable)
• [ ] I have added tests to cover my changes.

Priority:

• [x] Normal
• [ ] High

Related Issues:

Addresses #117

[codecov[bot]]

Codecov Report

Merging #46 into develop will not change coverage by %. The diff coverage is n/a.

@@            Coverage Diff            @@
##           develop       #46   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines          230       230           
  Branches        18        18           
=========================================
  Hits           230       230           

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