Separate link generation into individual classes

Problem/motivation

The Spatie Calendar Links library uses separate Generator classes for the calendar solutions it supports. This potentially makes it easier to support new solutions, tweak the logic for specific solutions already supported, and so on.

Proposed resolution

Refactor this module's current logic to use separate generator classes, likely using a base class and/or trait to reduce the amount of code needed for each specific implementation.

This could also effectively represent an API for sites to support additional calendar solutions.