Function angleRadian

angleRadian(a: Point, b?: Point, c?: Point): number
Returns the angle in radians between a and b.

Eg if a is the origin, and b is another point, in degrees one would get 0 to -180 when b was above a. -180 would be b in line with a. Same for under a.

Providing a third point c gives the interior angle, where b is the middle point.

See also angleRadianCircle which returns coordinates on 0..Math.Pi*2 range. This avoids negative numbers.
Parameters
- a: Point
- Optionalb: Point
- Optionalc: Point
Returns number
Example: Calculate angle between a middle of canvas and the cursor
```
const canvasEl = document.querySelector('canvas');
const middle = { x: canvasEl.width/2, y: canvasEl.height /2 }

canvasEl.addEventListener(`pointermove`, event => { 
 const cursor = {
   x: event.offsetX,
   y: event.offsetY
 }
 const a = G.Points.angleRadian(middle, cursor);
});
```
- Defined in geometry/src/point/angle.ts:36