Improve Corner Colors - BLANK enum and restore on reset

[sjtrny]

Change Summary:

1. Color definitions have been centralised
2. Modified the BLANK constant to have a non None value to resolve equality checks
3. The world corner_colors dictionary is now auto-filled with BLANK when corner color not provided
4. Painting BLANK corner color is now supported
5. Initial corner color state is restored when calling reset_world

Added associated tests for new functionality.

This PR resolves the following issues:

1. BLANK Corner Color #74
2. Corner Colors Not Restored #75

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.

Project coverage is 42.22%. Comparing base (28e34b2) to head (eb5d1ac).
Report is 14 commits behind head on main.

❗ Current head eb5d1ac differs from pull request most recent head 005e6db

Please upload reports for the commit 005e6db to get more accurate results.

Files	Patch %	Lines
stanfordkarel/karel_world.py	95.65%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
+ Coverage   41.47%   42.22%   +0.75%     
==========================================
  Files          10       10              
  Lines        1372     1376       +4     
==========================================
+ Hits          569      581      +12     
+ Misses        803      795       -8     

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