Update typography classes to be consistent

[owenatgov]

What

Update our typography classes to use the same sizes between lettered and numbered sizing. Eg:

Currently, govuk-body-s maps to 16 on the font scale whereas govuk-heading-s maps to 19 on the font scale. With this change, both classes would map to 19, meaning that govuk-[typography]-[size] would always have a consistent, predictable size.

Why

By keeping our sizing consistent across our typography, we make it easier to use and maintain.

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

Who needs to work on this

Developers