[css-shapes-1] Define `<basic-shape>` with CSS value definition syntax

I would like to get [`<basic-shape>`](https://drafts.csswg.org/css-shapes-1/#typedef-basic-shape) defined using the CSS value definition syntax so that it can be be extracted by `w3c/reffy`, similarly as [`<color>`](https://drafts.csswg.org/css-color-4/#typedef-color) is defined with its sub-productions.

> `<basic-shape> = <basic-shape-rect> | <circle()> | <ellipse()> | <polygon()> | <path()>`