Move Aspect Ratio code from Grid to Layout

[joshblack]

Transfer Aspect Ratio code (variables, mixins) from the Grid package over to Layout

• Move Sass values over to Layout and make sure that they're still called in Grid
  • Add test to packages/grid/__tests__/mixins-test.js for carbon--aspect-ratios and carbon--aspect-ratio mixin to check for existence
  • Add scss/_aspect-ratio.scss file over to @carbon/layout and include Variables and Mixin
  • Remove these values from grid mixin file
  • Add import for aspect ratio to grid mixin file
• Update layout/scss/layout.scss to include aspect ratio file

Add JavaScript values for aspect ratios and use in Layout package

• Add the aspectRatios exports to src/index.js
• Use the aspectRatios exports in tasks/build.js to generate a generated/_aspect-ratio.scss file
• Import the scale into scss/_aspect-ratio.scss

Original

```scss $carbon--aspect-ratios: ( (16, 9), (9, 16), (2, 1), (1, 2), (4, 3), (3, 4), (1, 1) ); ```

JavaScript

```js const unstable_aspectRatios = [ [16, 9], [9, 16], [2, 1], [1, 2], [4, 3], [3, 4], [1, 1], ]; ```

[joshblack]

cc @photodow would be interested in tackling this 👀

[photodow]

@joshblack yeah, let me see what I can do.