chore: rename module to @prometheus/client_js

[cjihrig]

This commit updates the project name from prom-client to @prometheus/client_js to reflect the recent move.

This is my first time contributing to this project under any package name, so please let me know if anything here is incorrect.

[slagiewka]

I know that @krajorama mentioned client_js as the desired state in #750 (comment). But this might've been intended to point to the repository name, not package name.

Reality check:
Python: pip install prometheus-client
Ruby: gem install prometheus-client
Rust: cargo add prometheus-client

Go is of course an outlier since it imports directly from domain's namespace. And in Go packages with _ in name are more commonplace.

prometheus-client is taken on npm. @prometheus/client would be more aligned with the npm ecosystem.