feat: add WEBPACK_SERVE env variable

[snitin315]

• 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

Replaced by WEBPACK_SERVE in webpack-cli.

Breaking Changes

replace WEBPACK_DEV_SERVER with WEBPACK_SERVE.

Additional Info

no

[alexander-akait]

No need to remove it, because some developers uses webpack-dev-server directly, let's rename to to WEBPACK_SERVE

[codecov]

Codecov Report

Merging #3000 (d238743) into master (92a8aeb) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3000   +/-   ##
=======================================
  Coverage   92.39%   92.39%           
=======================================
  Files          37       37           
  Lines        1262     1262           
  Branches      326      326           
=======================================
  Hits         1166     1166           
  Misses         91       91           
  Partials        5        5           

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

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[snitin315]

Something wrong with webpack-4 CI 😞, investigating.

[alexander-akait]

@snitin315 Thanks for help, if you want the next PR - we need change static https://github.com/webpack/webpack-dev-server/blob/master/lib/utils/normalizeOptions.js#L18 to public (some tests need to change too, but it is very easy)

[snitin315]

Will do next 👍.

[hiroppy]

hm, e2e seems fleky.

[alexander-akait]

@snitin315 CI cache broken, we need install webpack v4

[alexander-akait]

Thanks