Add themeable option to colour props for Heading component

[kwnath]

Overview

• Allows the usage of theme tokens to be used in the colour prop of the Heading component.
• Add a test to test the usage

Fixes #1242

This is likely going to be needed for other components so maybe good to think about how we do this in the future?

Testing

Create a component Heading with a theme token for the colour prop

<Heading color="blue500">Hello World</Heading>

Screenshots (if applicable)

Documentation

• [x] Updated Typescript types and/or component PropTypes
• [x] Added / modified component docs
• [x] Added / modified Storybook stories

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