Fix font names

- [new] Normalise variable and mixins names
    - `$sans-serif` → `$f-data`
    - `$serif` → `$f-serif-text`
    - `$serifheadline` → `$f-serif-headline`
    - `$text-sans` → `$f-sans-serif-text`
    - `$headline-sans` → `$f-sans-serif-headline`
    - `f-textsans()` → `f-textSans()`
    - `fs-textsans()` → `fs-textSans()`
- [new] Use the real font names:
    - AgateSans → Guardian Agate Sans 1 Web
    - EgyptianText → Guardian Text Egyptian Web
    - EgyptianHeadline → Guardian Egyptian Web
    - TextSans → Guardian Text Sans Web
    - HeadlineSans → Guardian Sans Web
- [new] Add 14/18 size to the Headline scale
- [new] Remove 14/12 size from the Body Copy scale
- [new] `guss-typography-defaults` mixin
- [new] Re-architect in multiple partials

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