Improve CSS integration with Gutenberg

[koke]

While developing WordPress/gutenberg#14826 we encountered some issues with CSS and Gutenberg:

1. Trying to reuse Gutenberg's _variables.scss didn't work, because some of the syntax wasn't accepted in react native (I'm not sure if it was React Native or the sass transformer).
2. Keeping the different syntax on a native-specific file didn't work either, since stylelint tried to enforce a syntax that wouldn't work on native

From WordPress/gutenberg#14826 (comment)

Having to set a font family via JS is counter-intuitive at the moment since it doesn't follow the rest of the code (setting styles via SCSS) so, it will be best if we track it.

Leaving the discussion of whether we should use CSS for native, as long as we do, there are a couple approaches I can think of:

• TIL that you can tell stylelint to ignore a certain rule in a specific CSS file, which would have solved the second issue.
• Have stylelint ignore .native.scss files, or apply a different set of rules.
• I think the sass-transformer we are using supports checking the platform in media queries, so we could have platform-specific CSS in the same file as the web. I don't know if stylelint would still try to validate that.