Fix public api for lab extension

[fcollonval]

The update environment was not part of the public interface. And so it was not accessible to third-party extension using the conda manager token.

In addition the token and the extension public interfaces have been moved to a tokens.ts file to respect JLab convention and minimize bundle size when jupyterlab_conda is an optional dependencies (i.e. it needs to be set as classical node dependency but webpack will only keep tokens.ts file in the bundle).