feat(assignees): add 6 new options to avoid stale for assignees

[C0ZEN]

New options (6):

• exempt-assignees
• exempt-issue-assignees
• exempt-pr-assignees
• exempt-all-assignees
• exempt-all-issue-assignees
• exempt-all-pr-assignees

Changelog:

• Add new function generateIIssue to easily generate a mock of IIsue
• Add new npm script test:only-errors
• Add new npm script test:watch
• Add new npm dependency jest-silent-reporter to avoid flood when tests are passing
• Refactor the Issue class to use getter instead of calculated properties in the constructor (more logical and reusable so that when the data change the getter will hold the new state without extra code)
• Add a new special syntax "$$type" in the IssueLogger to easily display either it's an issue or a pull request in the log without coding the logic each time reducing the complexity and the duplicated code
• Remove in some logs the issue/pr id since the logs already prefix the row with the id
• Add "user-friendly and understandable" logs for the assignees (IMO)
• Improve the contributing file

Closes #271
Closes #236