added guards for browser globals

[stevejpurves]

This draft PR illustrates the changes needed to allow jupyterlab to be statically included in remix.run application and bundled within server code. relates to issue #470

The changes here obviously may not leave the application in a functional state or apply good defaults, but they enable bundling / building and subsequent use in client-only parts of a remix / next.js based web application.

Hopefully these are useful as a guide in how to update / change intiialisaiton of some parts of lumino so there there is no dependency on browser globals at import time.

Note: these changes were determined from a specific use case, where domutils was pulled in as a dependency, there may be other similar issues in other lumino packages that need to be considered.

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