Implement new font method in Headings component

[mgurgel]

Blocked by #342

Is your feature request related to a problem? Please describe.
World Service has a UX requirement that GEL Types (Canon, Long Primer, Double Pica, etc) should allow for mutable values across services. These values were grouped into 11 different scripts: arabic, arabic-pashto, bengali, burmese, devanagari-and-gurmukhi, latin-and-cyrillic, latin-with-diacritics, no-ascenders-or-descenders, sinhalese, tamil, and thai.

The latin-and-cyrillic script contains the default values GEL for all the Types. The previous constants (GEL_CANON, GEL_LONG_PRIMER, GEL_DOUBLE_PICA, etc) are being deprecated in favour of script-specific methods such as getCanon(), getLongPrimer() and so on. These methods take the current component's script as an argument.

This issue is part of a wider work to update all existing components to use the new methods, eventually removing the old constants from the codebase.

Describe the solution you'd like

• Allow the component to pass the script name and children as a prop.
• Update the component's propTypes and defaultProps to include a prop of script that defaults to latin
• Import the appropriate getTypeSizes() method for the component's Type Styles (getCannon for Headline, getTrafalgar for Subheading)
• Implement the method above, replacing any references to the deprecated GEL Type constant (GEL_CANON, GEL_TRAFALGAR) in the styled-component CSS string
• Remove existing imports of the previous GEL Type constant

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"

[sareh]

Prior to the work being done for adding the script props to the front-end components such as this one, could you please have a short write-up in an ADR explaining the approach? Thanks!

[alex-magana]

As indicated above #344 had been closed in favour of this issue since the work for Headline and SubHeading is to be done in the psammead-headings component.
cc @jblaut