Extract vectorish module into its own package

[hmans]

Sometimes libraries (like this one!) are designed to be used alongside Three.js, but don't want to add it as a dependency just to make use of its Vector2 class (because they might be used with something that is not Three.js.) The vectorish module contains some lightweight 2D vector math that might be suitable for extraction into a separate package, so these libraries can just add that and be happy.