Extract MathJaxTypesetter into library

[vidartf]

It would be nice to be able to import/use the MathJaxTypesetter without getting a dependency on @jupyterlab/application. This could be achieved if the class definition was extracted out from the extension package, into a separate library package.

On a similar note, would it be reasonable to move materialcolors.css from @jupyterlab/application to @jupyterlab/apputils? As far as I can tell from the dependency map at https://jlab.readthedocs.io/en/debug-rtd/extensions_dev.html, this should not add any additional dependencies.

[vidartf]

PS: If there is consensus for this, I'd be happy to do the legwork in a PR.

[blink1073]

Sounds good Vidar, yes, please!

[vidartf]

@blink1073 On both counts (typesetter and materialcolors)?

[blink1073]

Yup!

[vidartf]

Actually it should be both, otherwise apputils has an implicit dependency on application (otherwise there will be missing CSS vars, as e.g. commandpalette.css uses md vars).