Refactor Icon base class to be extendable

[rajsite]

Pull Request

🤨 Rationale

Refactor the Icon base class so new types of icons can extend the class manually. Should be a non-breaking change for users as existing icon api and paths don't change. Intention is to support custom icon types such as multicolor icons (#327) and dynamic icons (#2813).

👩‍💻 Implementation

• Refactor the Icon base class to be abstract
• Make a new SVGIcon base class that implements the current infrastructure for nimble icons

🧪 Testing

• Rely on existing tests

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.