Make it easy to build polygonal faces

[hannobraun]

There are straight-forward methods in the operations API to build polygonal Cycles and Regions. However, for Faces, there is only this:

fornjot/crates/fj-core/src/operations/build/face.rs

Lines 23 to 58 in c29e2a3

	/// Build a triangle
	fn triangle(
	points: [impl Into<Point<3>>; 3],
	services: &mut Services,
	) -> Polygon<3> {
	let [a, b, c] = points.map(Into::into);
	let surface = Surface::plane_from_points([a, b, c]).insert(services);
	let (exterior, edges, vertices) = {
	let half_edges = [[a, b], [b, c], [c, a]].map(|points| {
	let half_edge = HalfEdge::line_segment_from_global_points(
	points, &surface, None, services,
	);
	half_edge.insert(services)
	});
	let vertices = half_edges
	.each_ref_ext()
	.map(|half_edge| half_edge.start_vertex().clone());
	let cycle = Cycle::new(half_edges.clone()).insert(services);
	(cycle, half_edges, vertices)
	};
	let region = Region::new(exterior, [], None).insert(services);
	let face = Face::new(surface, region);
	Polygon {
	face,
	edges,
	vertices,
	}
	}
	}

This method could be generalized to build polygons instead of just triangles, but there is one thing to figure out: This method takes 3D points (since a Face is a 3D object), and exactly 3 points define a plane. If generalizing this method for polygons, we have to handle the case that the user provides points that don't lie in the plane defined by the first three points.

[hannobraun]

Addressed in #1912.