fix: schema for server option

[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

server can not be any string because we use it to require a node module.

webpack-dev-server/lib/Server.js

Line 1591 in 6d060ed

this.server = require(this.options.server.type).createServer(

Breaking Changes

No

Additional Info

No

[codecov]

Codecov Report

Merging #4000 (7ac9d6b) into master (2b42431) will decrease coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4000      +/-   ##
==========================================
- Coverage   92.74%   92.60%   -0.15%     
==========================================
  Files          14       14              
  Lines        1406     1406              
  Branches      516      516              
==========================================
- Hits         1304     1302       -2     
- Misses         94       96       +2     
  Partials        8        8              

Impacted Files	Coverage Δ
lib/servers/WebsocketServer.js	91.42% <0.00%> (-5.72%)	⬇️

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[alexander-akait]

In theory developer can use type: "own-server-package" which compatibility with connect, so let's add test for this case (just reexport express for tests)

[alexander-akait]

Thanks