Update public path logic

[vidartf]

Following https://ipywidgets.readthedocs.io/en/stable/migration_guides.html#updating-the-webpack-publicpath-configuration

CC @maartenbreddels

Fixes #395

[codecov-commenter]

Codecov Report

Merging #398 (fa869a4) into master (6102f0c) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #398   +/-   ##
=======================================
  Coverage   70.54%   70.54%           
=======================================
  Files          23       23           
  Lines         893      893           
=======================================
  Hits          630      630           
  Misses        263      263           

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more