Add aspect ratio dimensions value (tailwindlabs#6178)

* Add aspect ratio `dimensions` value

* Update defaultConfig.stub.js

Co-authored-by: Adam Wathan <adam.wathan@gmail.com>

Author: neupauer

stubs/defaultConfig.stub.js

@@ -115,6 +115,7 @@ module.exports = {
       auto: 'auto',
       square: '1 / 1',
       video: '16 / 9',
+      attrs: 'attr(width) / attr(height)'
     },
     backdropBlur: ({ theme }) => theme('blur'),
     backdropBrightness: ({ theme }) => theme('brightness'),