Fix conflict when using field-grid-dropdown with field-colour

[berkerozer]

🐛 Fix: Register Conflict Between @blockly/field-grid-dropdown and @blockly/field-colour

This PR addresses an issue where using @blockly/field-grid-dropdown and @blockly/field-colour together would cause a Name "field_grid_dropdown" already registered error due to double registration.

🔧 Changes Made

• Introduced a new utility function registerFieldGridDropdown() in @blockly/field-grid-dropdown.
• Modified @blockly/field-colour to import and call registerFieldGridDropdown() explicitly.
• Ensured that the dropdown field is only registered once and safely when needed.

✅ Benefits

• Avoids duplicate field registration errors.
• Improves modular use of Blockly plugins.
• Safer integration of multiple plugins across different projects.

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