ci: Add @graphprotocol/gnd npm wrapper package

Add a wrapper package that installs the correct platform-specific gnd
binary via optionalDependencies. This enables `npm install -g
@graphprotocol/gnd` and `npx @graphprotocol/gnd`.

The wrapper includes:
- gnd/npm/bin/gnd.js: launcher script that resolves the platform binary
- gnd/npm/README.md: package documentation
- publish-npm-wrapper workflow job that runs after platform packages