Add Contrast Axis

[felipesanches]

Fonts to be onboarded that have this axis:

• https://github.com/google/fonts/issues/2931

---

@yanone originally posted this at https://github.com/google/fonts/issues/4355:

---

I'm proposing the inclusion of a "contrast" axis (CNTR) into the axis registry. https://github.com/tallchai/akshar-type has one, that I’m currently onboarding.

tag: "CNTR"
display_name: "Contrast"
min_value: 0.0
default_value: 0.0
max_value: 100.0
precision: 1
fallback {
  name: "Low"
  value: 0.0
}
fallback {
  name: "High"
  value: 100.0
}
description:
  "Stroke contrast describes the stroke width difference"
  " between the thick and thin parts of a pen stroke."
  " Traditionally, a Redis pen nib would yield a low contrast stroke"
  " (monolinear) while a broad nib or pointed nib pen would"
  " yield a high contrast stroke."

cc @m4rc1e @davelab6 @rsheeter