fix: expose app entrypoint in webpack dev config for example app

[adamstankiewicz]

Currently, running npm start on a fresh checkout results in a blank example app running on localhost:8080. This is a result of the recently merged ParagonWebpackPlugin changes in @openedx/frontend-build which assumes MFEs define an app entrypoint, as is the case in the default webpack.dev.config provided by @openedx/frontend-build, which this repo's example app overrides to be non-standard.

This PR defines the app entrypoint in webpack.dev.config.js.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.60%. Comparing base (45e551e) to head (6df74d6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #531   +/-   ##
=======================================
  Coverage   70.60%   70.60%           
=======================================
  Files          24       24           
  Lines         364      364           
  Branches       96       96           
=======================================
  Hits          257      257           
  Misses        105      105           
  Partials        2        2           

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