Move `weak-node-api` into its own package

[kraenhansen]

It seems the concept is useful to other projects and for use across the ecosystem, I think I'd be better if the weak-node-api library was separated out of the host package:

• packages/host/scripts/generate-weak-node-api-injector.ts
• packages/host/scripts/generate-weak-node-api.ts
• packages/host/weak-node-api