Remove need for CustomHandler annotations

[ifiokjr]

Description

The CustomHandler is pretty ugly.

When setting up an extension you can define options as Handlers and CustomHandlers. Handlers are designed to capture functionality like onChange and onExit. Behind the scenes a handler is just an array that tracks the number of functions that have been added. When using extension.options.onChange(value) the handler will run each of the methods in the handler method array. The handler can also be configured to exit early based on a certain return value.

CustomHandler's were created for situations which needed more customisation. For example, keyBindings. A keybinding is an object with each key representing a keyboard combination and the value a method that should be called when the combination is active.

It's not obvious how you would compose multiple keyboardBindings together and the custom handler was created to enable this situation. However, there are some issues. You no longer have the extension.options.keyBinding instead it's up to you how to compose things together.

I want to explore whether there might be easier ways of composing the functionality in an automated way as part of the Handler option.

[ifiokjr]

After #632 it’s clear to me that there will always be a need for custom handlers and the implementation is too involved to play around with.