Separate manager and preview part.2

[ndelangen]

CHANGE build & dev-serve so both webpack compilation happen in the same process
CHANGE dev-serve so it watches manager

Ease of development on storybook has taken a hit, this changes the manager build back into the single (main) serve/build process.

On dev the manager build is now in watch mode.

[codecov]

Codecov Report

Merging #4789 into next will increase coverage by 0.08%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##             next    #4789      +/-   ##
==========================================
+ Coverage   35.47%   35.55%   +0.08%     
==========================================
  Files         561      560       -1     
  Lines        6825     6809      -16     
  Branches      911      910       -1     
==========================================
  Hits         2421     2421              
+ Misses       3928     3913      -15     
+ Partials      476      475       -1

Impacted Files	Coverage Δ
lib/core/src/server/build-static.js	0% <0%> (ø)	⬆️
lib/core/src/server/dev-server.js	0% <0%> (ø)	⬆️

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