Set env vars for `jupyter-book` whitelabelling in TS

[agoose77]

Presently the JB package on PyPI whitelabels properly, whilst the JS package does not. Both should be whitelabelled, with the NPM package referring to itself as jupyter-book. Consequently, we might also add a binary output to the NPM package.

[rowanc1]

@agoose77 did we decide against this approach in person?

[agoose77]

I think worth a revisit. The argument for whitelabelling was binary reuse. We might still need a way to pass the app name all the way down to deep parts of myst CLI, but that probably points to us defining that state on the session.