lib: clean up PR and CI url parsing logic

[joyeecheung]

• Put the PR URL parsing logic in links.js and reuse it
• Differentiate PR and commit jobs in preparation for ncu-ci: command that shows results of jenkins #161
• Updates a few CI link patterns

[codecov]

Codecov Report

Merging #242 into master will decrease coverage by 0.03%.
The diff coverage is 94.59%.

@@            Coverage Diff             @@
##           master     #242      +/-   ##
==========================================
- Coverage   88.55%   88.52%   -0.04%     
==========================================
  Files          19       19              
  Lines         708      723      +15     
==========================================
+ Hits          627      640      +13     
- Misses         81       83       +2

Impacted Files	Coverage Δ
lib/links.js	100% <100%> (ø)	⬆️
lib/pr_checker.js	98.4% <100%> (ø)	⬆️
lib/ci.js	93.02% <91.66%> (-4.2%)	⬇️

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[joyeecheung]

cc @nodejs/automation

[priyank-p]

The cleanup looks great.