Expose webpackFinal extension

[igor-dv]

Issue:
angular-cli preset should filter out all the styling rules in order to integrate its rules instead.
Currently, angular-cli preset is kicked-in in the middle of the presets chain because it's a framework preset.

The last preset in the chain adds default rules or execute the custom webpack config. We need the ability for other presets to clean the webpack config after all the manipulations.

Related #4296

What I did

I've added the webpackFinal step.

[codecov]

Codecov Report

Merging #4431 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #4431      +/-   ##
==========================================
- Coverage   35.98%   35.97%   -0.02%     
==========================================
  Files         555      555              
  Lines        6650     6652       +2     
  Branches      871      871              
==========================================
  Hits         2393     2393              
- Misses       3810     3812       +2     
  Partials      447      447

Impacted Files	Coverage Δ
...angular/src/server/framework-preset-angular-cli.js	0% <ø> (ø)	⬆️
lib/core/src/server/config.js	0% <0%> (ø)	⬆️

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[igor-dv]

Looks like it should also fix the initial issue from #4230

[kroeder]

Looks fine, thanks for clarifying presets