Bidirectional slant axis support for variable fonts

@Gue3bara recently updated the [upstream repository](https://github.com/Gue3bara/Cairo) for the the typeface [Cairo](https://github.com/Gue3bara/Cairo) with a bidirectional slant axis. See the FontGoggles preview below.


The reasoning behind this was posted in the [Cairo PR](https://github.com/google/fonts/pull/2380#issuecomment-603342967).

>I thought it might be unusual but I want the font to support both slanting directions to be available as an option depending on the main language of the paragraph if it's Latin featuring Arabic words or Arabic featuring Latin words I think it is essential to support both directions for bi-script font families like Cairo

I know that Google Fonts is not able to serve `slnt` axis fonts yet, due to lack of [browser](https://github.com/arrowtype/recursive/issues/56) support, but when more support for the `slnt` axis is added I want to suggest that bidirectional support is included in that.