Accepts the gradient definitions as it is declared in `background-image` and returns an AST `object`.
## AST
### Common properties
All nodes have the following properties.
#### type
`String`. The possible values are the ones listed in the Types section bellow.
### Types
The available values of `node.type` are listed below, as well as the available properties of each node (other than the common properties listed above).
### linear-gradient
- orientation: `Object` possible types `directional` or `angular`.
- colorStops: `Array` of color stops of type `literal`, `hex`, `rgb`, or `rgba`.
### repeating-linear-gradient
- orientation: `Object` possible types `directional` or `angular`.
- colorStops: `Array` of color stops of type `literal`, `hex`, `rgb`, or `rgba`.