Fix CSS source maps

[agubler]

Type: bug

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier
• Unit or Functional tests are included in the PR

Description:

Always use optimize-css-assets-webpack-plugin, not just in dev mode. Pass options to ensure that source-maps are preserved.

Resolves #218

[codecov]

Codecov Report

Merging #225 into master will decrease coverage by 0.95%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
- Coverage   53.64%   52.68%   -0.96%     
==========================================
  Files          10       10              
  Lines         604      596       -8     
  Branches      147      144       -3     
==========================================
- Hits          324      314      -10     
- Misses        280      282       +2

Impacted Files	Coverage Δ
src/dist.config.ts	32.2% <ø> (-1.13%)	⬇️
src/base.config.ts	22.85% <100%> (+0.89%)	⬆️
src/main.ts	90.58% <0%> (-1.6%)	⬇️

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