reference implementation for a file browser

[tonyfast]

the approach to having an html5 reference for notebook representations has proven informative to jupyterlab development. the file tree is another critical component that could use a reference implementation to guide development. this is relevent to ongoing discussion in jupyterlab jupyterlab/jupyterlab#15933

this is an interesting applications because the flexible table roles we developed for notebooks is applicable to different contexts of the file browser in jupyterlab and notebook. the file browser is the only major component on the page in the notebook application. while in jupyterlab, different roles will be necessary since there are other points of interest on the page.

nbviewer has a concept of a file browser further encouraging us to improve the assistiveness of that experience. this will be quicker to prototype with our improved understanding of table accessibility in notebook contexts.