All those other engines are cross-platform, whereas LibJS is UNIX only and actually very dependent on SerenityOS libraries (so not really a portable UNIX library), but that's apparently the intended design.
To be fair both JavaScriptCore and V8 heavily rely on the base C++ libraries that are part of Chromium and Webkit.
You can totally build and use V8 independently (many projects, including node.js, do it) and they have documentation specifically targeted at embedders.
JSC is not as nice but can still be built and embedded as a stand-alone library.
Yes but in doing so you will also build the supporting C++ library. I imagine it would be relatively easy to do the same for the serenity javascript engine / browser. The serenity standard library https://github.com/SerenityOS/serenity/blob/master/Meta/Lago... has support for other platforms.