v0.4.0

This changes y-rb to use yrs 0.13.x+. Even this being a change which does not add any new features for Ruby users, it unfortunately includes a few breaking changes. I tried to keep them to a minimum, but yrs behaves different in certain situations, and it cannot be avoided completely.

For the best reference on what kind of breaking changes you should expect, please check the changes in y/specs here b270b78.