Function rotateByVertex

rotateByVertex(
    triangle: Triangle,
    amountRadian: number,
    vertex?: "a" | "b" | "c",
): Triangle
Rotates the vertices of the triangle around one point (by default, b), returning as a new object.
```
let triangle = Triangles.fromPoints([a, b, c]);
triangle = Triangles.rotateByVertex(triangle, Math.Pi, `a`);
```
Parameters
- triangle: Triangle
  Triangle
- amountRadian: number
  Angle to rotate by
- vertex: "a" | "b" | "c" = ...
  Name of vertex: a, b or c.
Returns Triangle
A new triangle