feat(exempt): add new options to exempt the milestones

[C0ZEN]

Changes:

• Add new options exempt-milestones, exempt-issue-milestones and exempt-pr-milestones
• Undo the changes in the lint scripts to include Prettier
  • Did not saw that some scripts were already there to run Prettier
  • Instead create new scripts to run Prettier and ESLint (which is very useful for me at least)
• Create an Issue class to contains all the related calculated data
  • Easier to use the calculated data
  • Make more sense in OOP world IMO
• Create an Milestones class to handle all the logic regarding the milestones
• Fix some small spellcheck issues
• Renamed some models/functions
  • Some models were prefixed with I since it's an interface related to a class
  • The interface is clearly in that case an API model; can keep this convention if you are ok

Closes #270.
Partially closing #272.
Partially closing #236.

[C0ZEN]

@hross if you have some time left, I would love to get some help to find the culprit regarding the failing check 😢

Edit:
I think it's related to some config options being optional but yet parsed as required numbers.
Need to update a bit the code here.

Edit 2:
The culprit was indeed optional number values and was not coming from this PR.
This was due to the fact that building the index was required in order to make the checks good and up-to-date.
IMO the index build should be inside the CI and should be ignored from the repo.