Note:
It was created from #290 so it would be better to merge it first.
It would be cooler if #289 can be merged before too so that the beta or alpha release can be tested.

Changes:

• Create more spec files to test the general behaviour of the stale action
• Move into their own files the utils code to test the general behaviour of the stale action
• Refactor the tests for the milestones
  • Add coverage for issues AND pull requests as it should
  • Create a more nested structure to be more logical and make sure no cases was forgotten
  • Note: damn this is a 10k rows kind of spec file... since there is now 6 options playing altogether with the milestones there is a bunch of possible config combinations and I am pretty sure to cover them all at this cost. Any suggestion is welcome
• Create a tsconfig for the spec and for the app to make sure eslint AND jest find the right files at any moment
• Rename the pr wordings to make it more consistent and more user-friendly
• Add new options to ignore all issues and PRs with milestones with their respective sub-options to do that only for issues OR pull requests - the option has a better priority than the current exempt milestones options

Closes #283