Implement script prop in Subheading component

[mgurgel]

Blocked by #342

Is your feature request related to a problem? Please describe.
Subheading component must adopt the script prop to allow for theming

Describe the solution you'd like
Component is aware of and uses the script prop in its code

Testing notes
[Tester to complete]

Dev insight: Will Cypress tests be required or are unit tests sufficient? Will there be any potential regression? etc

Additional context
Add any other context or screenshots about the feature request here.

• Initially labelled with "Refinement needed"

[mgurgel]

Work for this ticket has been moved to #343