The plugin system should be reworked #243
Description
Instead of having different kinds of plugins the plugin system should be rewritten to support more generic plugins using a "hook" based system. For instance Astro's "integrations" feature is an inspiration.
With such a plugin system it's much easier to extend Emigrate's functionality without having to invent new types of plugins all the time. A plugin can also decide what parts to implement and has more control than the current solution.
With a hook based system it's also easy to solve #169