[WIP]: Use class inheritance for plug-in API

[paulrobertlloyd]

By using class inheritance with a base IndiekitPlugin class with child classes for the different plug-in types, it becomes possible to orchestrate common functionality and centralise debugging and error handling.

This becomes even more important if we want to use an event-based model with hooks, to allow multiple plugins (and of certain types) to perform functions at given points in the publishing lifecycle. The goal is to strike a balance between giving plug-ins flexibility to determine behaviour, but have a plug-in system that works in a reliable and consistent way.

We should try to keep largely to the existing plug-in API, but at least one required changed would be to have a plug-in inherit from a parent plugin class to continue working.

Todo

• Initial proof of concept
• Convert endpoint plug-ins
• Convert post type plug-ins
• Convert preset plug-ins
• Convert store plug-ins
• Convert syndicator plug-ins
• Add checks for required properties and methods
• Built-in error handling
• Update tests
• Add tests for base and type plug-ins
• Add ADR
• Update documentation

Later PRs

• Automatic plug-in registration?
• Publication presets set post/media paths without directly modifying postTypes object