Is Chrome Dev Tools available only through Chrome extensions? Or is there a way we can use it, for instance, as a library from a JS standalone file?
Chrome DevTools Frontend is now an NPM package that you can take advantage of. It is based from the new DevTools sub-repo in Blink. So you can easily pull in the DevTools assets and keep then up-dated without much work.
It depends on what you want exactly. You can dig in and rip out bits of the frontend to use. Like Betwixt by Konrad pulls the network panel design out and plugs it up to an OS interceptor. So you can use the network panel you know and love to debug any app on your desktop.
Now, can you easily do this kind of stuff. Even keeping it auto-updated? No, not really. It is fairly complicated and you need to be able to handle ripping the stuff out yourself. That being said, if you want a challenge then the code is open to everyone.