fonts/recursive-ttf: add "code" variant of the fonts.

These have code ligatures & several stylistic sets pre-applied so they
will work by default in most code environments & apps. These are also
named & configured in a way that enables their use in code themes that
utilize italic & bold styles. If you wish to configure specific features
in Rec Mono fonts, see
Recursive Code Config https://github.com/arrowtype/recursive-code-config

- Regular, Italic, Bold, & Bold Italic static fonts
- Frozen-in Code Ligatures
- An abbreviated family name to enable italic themes on macOS
- Reduced-slant italics for easier readability in code (normal Recursive
  Italics have slnt=-15, which is pretty intense)
- Frozen-in OpenType features to enhance legibility for code (e.g.
  making 1 and l instantly recognizable)
  - `ss03` # simplified f
  - `ss05` # simplified l
  - `ss08` # serifless L and Z
  - `ss09` # simplified 6 and 9
  - `ss12` # simplified @

(rhialto)