Add GDS styling for code snippets

[daniel-ac-martin]

See: https://govuk-prototype-kit.herokuapp.com/docs/install/install-the-kit.md

[daniel-ac-martin]

(Thanks to @tomyems for the suggestion.)

[tomyems]

Would code snippets be 'components-internal' rather than 'components'? I guess it depends on whether applications are likely to use code snippets in production?

There are two styles to code snippets in the GOV.UK prototype. One is inline in red, and the other is in a scrollable block. Should they be one component, with an attribute that toggles between inline or block, or are they two separate components? (I'd have guessed that they're doing the same function so in one component?)

[daniel-ac-martin]

I think, for now, just some default styling on <pre> and <code> blocks would go a long way. I'm not exactly sure where that should live or whether it needs to be a component.

Then again, I've already got some stuff for the preview boxes on the documentation pages, so perhaps I could just generalise that... (Might be nice as there's some support for syntax highlighting.)