Fix custom webpack config warnings

[igor-dv]

Issue: Fix the #3981 + a bit more refactoring

What I did

I've refactored loadCustomWebpackConfig.js and now we have a serverRequire.js that will be able to load client's scripts based on their preferred compiler on server (Currently the only usecase is webpack.config, but I assume will be more, for example: presets, single config file, etc')

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@4121d26). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master    #4009   +/-   ##
=========================================
  Coverage          ?   38.79%           
=========================================
  Files             ?      444           
  Lines             ?     5589           
  Branches          ?      751           
=========================================
  Hits              ?     2168           
  Misses            ?     3026           
  Partials          ?      395

Impacted Files	Coverage Δ
lib/core/src/server/config/interpret-files.js	72.72% <0%> (ø)
lib/core/src/server/loadCustomWebpackConfig.js	0% <0%> (ø)
lib/core/src/server/serverRequire.js	0% <0%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4121d26...746c2e2. Read the comment docs.

[emmenko]

Any ETA on when this fix will be released? 😇