feat: onListening option

[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 #1569

Breaking Changes

No

Additional Info

• Also added deprecation for next major release
• This option allow to developers run own scripts after run webpack-dev-server

[codecov]

Codecov Report

Merging #1930 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1930      +/-   ##
==========================================
+ Coverage      88%   88.03%   +0.02%     
==========================================
  Files          16       16              
  Lines         817      819       +2     
  Branches      260      261       +1     
==========================================
+ Hits          719      721       +2     
  Misses         84       84              
  Partials       14       14

Impacted Files	Coverage Δ
lib/Server.js	86.02% <100%> (+0.06%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 87cfaa7...8bdbee8. Read the comment docs.

[codecov]

Codecov Report

Merging #1930 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1930      +/-   ##
==========================================
+ Coverage   91.11%   91.13%   +0.02%     
==========================================
  Files          18       18              
  Lines         833      835       +2     
  Branches      261      262       +1     
==========================================
+ Hits          759      761       +2     
  Misses         70       70              
  Partials        4        4

Impacted Files	Coverage Δ
lib/Server.js	91.55% <100%> (+0.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9297988...2466438. Read the comment docs.

[hiroppy]

need to add tests to options.test.js. https://github.com/webpack/webpack-dev-server/blob/master/test/options.test.js

[alexander-akait]

/cc @hiroppy done

[hiroppy]

Thanks