[chronik-client] Add prepublish command to prevent unbuilt npm deployments

Summary:
Depends on D14270

Package must be built before deployment. Adding 'prepublish' command so that npm publish will build the package before publishing.

Test Plan: Confirm spelling of "prepublish" and that npm run build builds the package

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien, emack

Differential Revision: https://reviews.bitcoinabc.org/D14271