Add missing function declarations to resistor_color.h

[user1-prajwal]

The header file currently contains an empty enum definition,
which may be confusing for contributors working locally.

Fixes #1095

This change adds:

• Enum values for resistor colors
• Function declarations for color_code() and colors()

This improves clarity and aligns the header with the expected API.

[ryanplusplus]

Hey @user1-prajwal, the intent is for the user to determine what should be in the header as part of the exercise.

[user1-prajwal]

Thanks for the Clarification! I misunderstood the intent of the exercise earlier. I appreciate you taking the time to explain it.