Calculate the area of a simple 2D polygon given its vertices.

This is a port of the math-utils/area-polygon library written in JavaScript.

## Usage

```
List<Offset> points = [
// your points here, e.g. a 4x4 square:
Offset(0, 0),
Offset(0, 4),
Offset(4, 4),
Offset(4, 0),
];
double area = calculateArea(points); // 16
print('Area: $area');
```

The `points`

must also trace the edge of the polygon.
The last point is assumed to be connected to the first point.

If `signed`

is true, this will return the signed area.
This is helpful to determine the orientation of `points`

.

Signed area is positive if the `points`

are counter-clockwise,
or negative if the `points`

are clockwise.