Add stop command to unmock-server.

[ksaaskil]

• Add unmock-server stop command
• Works by writing process PID to ~/.unmock/server.pid to file when unmock server is started and adding a SIGTERM handler. Stop command then reads the file and sends SIGTERM to the process.
• Alternative would be to add an endpoint in the servers such as "/stop" but implementing that is not the easiest thing for the proxy server. Or should one add an /api/stop endpoint in the mock server and also stop the proxy server there, coupling the two?
• Could something go wrong if the PID in server.pid points to the wrong process? This should not happen if start command gets to do its clean-up properly (defined in process.exit handler)

[codecov-io]

Codecov Report

Merging #374 into dev will decrease coverage by 0.04%. The diff coverage is 77.5%.

@@            Coverage Diff            @@
##              dev    #374      +/-   ##
=========================================
- Coverage   80.35%   80.3%   -0.05%     
=========================================
  Files          54      55       +1     
  Lines        2428    2468      +40     
  Branches      585     592       +7     
=========================================
+ Hits         1951    1982      +31     
- Misses        477     486       +9

Impacted Files	Coverage Δ
packages/unmock-server/src/server.ts	66.21% <0%> (-0.91%)	:arrow_down:
packages/unmock-server/src/pid.ts	79.48% <79.48%> (ø)

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