This bumps the canary versions to v19 to communicate that the next
release will be a major. Once this lands, we can start merging breaking
changes into `main`.
This lets us share it with react-server-dom-webpack while still having a
dependency on react-dom. It also makes somewhat sense from a bundling
perspective since react-dom is an external to itself.
Andrew Clark
Ricky
Sebastian Markbåge