fix: add a helper function in frontend-build to retrieve relative path from webpack to env.config.js

[jsnwesson]

APER-2786

This is a follow-up to #275 to provide the relative path of env.config.js being provided by the environment variable JS_CONFIG_FILEPATH from within webpack.prod.config.js. This will enable webpack to create a copy of env.config.js in Learner Record.

This work is a temporary patch until we can add these changes to frontend-build and Learner Record migrates from using @edx/frontend-build to @openedx/frontend-build as a dependency.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2aac86b) 70.22% compared to head (2df4b7b) 70.22%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #283   +/-   ##
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  Coverage   70.22%   70.22%           
=======================================
  Files          27       27           
  Lines         403      403           
  Branches       85       85           
=======================================
  Hits          283      283           
  Misses        119      119           
  Partials        1        1           

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[jsnwesson]

Closing this PR due to the fact that it overcomplicates the alternative solution of providing a relative path for env.config.js.