Fix 2D heightfield scale and docs

[Jondolf]

Objective

Fixes #342.

The 2D heightfield only has a scalar scale argument instead of allowing different scaling along each coordinate axis. The docs are also incorrect.

Solution

Take a Vec2 instead of a scalar value for the 2D Collider::heightfield and fix the heightfield docs.

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Migration Guide

The 2D Collider::heightfield method now takes a Vec2 scale instead of a single float. This way, it now supports non-uniform scaling.

// Before
let collider = Collider::heightfield(heights, 2.0);

// After
let collider = Collider::heightfield(heights, Vec2::new(2.0, 2.0));