Normalize should not check for arbitrarily small lengths

[deluksic]

I discovered a bug in my app because vec2.normalize was avoiding division by 0 by checking an arbitrarily small length:

https://github.com/greggman/wgpu-matrix/blob/main/src/vec2-impl.ts#L506

I believe it should only check for 0, by doing len || 1 for example and leave other numerical instabilities to the caller.

Situation in which this appeared is a demo app where you can zoom in a lot, so vectors become super small. That doesn't mean that these are not valid vector values that can be normalized.

(demo app in question https://deluksic.github.io/apollonian-circles/)