feat: borders

[JMBeresford]

Is your feature request related to a problem? Please describe.
In the DOM we can decorate the and <textarea> elements with a border; this should be possible with the respective 3D elements as well.

Describe the solution you'd like
Allow a border to be defined on <Input /> and <Textarea /> like so:

// @param borderWidth: size of border in terms of percentage of container size
// @param borderColor: color of the border
// @param borderOpacity: opacity of the border
<Input borderWidth={0.1} borderColor={{r: 0.5, g: 0.5, b: 0.5}} borderOpacity={0.75} />