fix: multi compiler mode with proxy

[alexander-akait]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

For Bugs and Features; did you add new tests?

Yes

Motivation / Use-Case

fixes #2891

Breaking Changes

No

Additional Info

it is dirtly fix, because developer can have two configs with the devServer option, but in v3 and v4 we have limitation - only one configuration can have devServer, otherwise we use 0 index for configurations, we will fix it after refactor it to plugin (I think v5 or v4 after stable release)

[codecov]

Codecov Report

Merging #2905 (d4ad655) into master (9e65c24) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2905      +/-   ##
==========================================
+ Coverage   92.52%   92.53%   +0.01%     
==========================================
  Files          37       37              
  Lines        1258     1260       +2     
  Branches      333      334       +1     
==========================================
+ Hits         1164     1166       +2     
  Misses         89       89              
  Partials        5        5              

Impacted Files	Coverage Δ
lib/Server.js	94.88% <100.00%> (+0.02%)	⬆️

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