Allow radius1 as an alias for radius in ol.style.RegularShape

When creating star symbols having radius1 and radius2 makes more sense as config options, hence the proposal to have radius1 be an alias for radius in ```ol.style.RegularShape```
The documentation would be more straightforward if we had the following distinction:

"Specify radius for regular polygons, or radius1 and radius2 for stars."
* radius: radius of a regular polygon
* radius1: inner radius of a star
* radius2: outer radius of a star