Update our font helpers to use a t-shirt sizing scale

[owenatgov]

What

Update our font sass mixins eg: govuk-font to allow for a "t-shirt" sizing, lettered scale on top of or instead of the numbered scale we current use.

Why

So it's easier for users of the font helpers to understand what they can do with the font mixins and what to expect when using them in any given context.

Part of https://github.com/alphagov/govuk-design-system/issues/2289

Who needs to work on this

Developers