Add a "code" primary language

Discussion with @simoncozens, @mariannapaszkowska, @m4rc1e + cc @davelab6

With **_Google Sans Code_**, we’re introducing for the first time a [sample text containing code](https://fonts.google.com/specimen/Google+Sans+Code).

It would be great to add similar samples for all current and future coding fonts. Even better would be to automate this process, instead of manually copy-pasting a code sample for each font.

The idea is to add "code" as a language, using this format:
`primary_language: "code_Latn"`

@rsheeter @evanwadams what do you think?