fix(plugin-webpack): fix deprecation warnings for dev-server@4.0.0

[malept]

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Summarize your changes:

It looks like between webpack-dev-server 4.0.0-rc.0 and 4.0.0, they added some API deprecations/breakage. This updates all that based on the migration docs.

Fixes #2456

[codecov]

Codecov Report

Merging #2457 (522ccdf) into master (bd2526b) will increase coverage by 2.34%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2457      +/-   ##
==========================================
+ Coverage   73.00%   75.35%   +2.34%     
==========================================
  Files          91       91              
  Lines        2349     2353       +4     
  Branches      440      440              
==========================================
+ Hits         1715     1773      +58     
- Misses        469      472       +3     
+ Partials      165      108      -57     

Impacted Files	Coverage Δ
packages/plugin/webpack/src/WebpackPlugin.ts	36.84% <0.00%> (ø)
packages/maker/base/src/Maker.ts	75.67% <0.00%> (-2.11%)	⬇️
packages/installer/darwin/src/InstallerDarwin.ts	31.57% <0.00%> (-1.76%)	⬇️
packages/api/core/src/util/upgrade-forge-config.ts	94.73% <0.00%> (-1.70%)	⬇️
packages/api/core/src/api/init.ts	97.56% <0.00%> (+4.87%)	⬆️
packages/api/core/src/api/make.ts	88.29% <0.00%> (+6.57%)	⬆️
packages/api/core/src/util/rebuild.ts	100.00% <0.00%> (+7.14%)	⬆️
packages/api/core/src/api/install.ts	94.87% <0.00%> (+16.66%)	⬆️
packages/api/core/src/api/start.ts	87.65% <0.00%> (+22.22%)	⬆️
packages/api/core/src/api/publish.ts	92.10% <0.00%> (+22.36%)	⬆️

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