hydra extension API [WIP]

[ojack]

Talking wih @jamiefaye about adding InAct as an extension to the hydra editor. I would like to use this as an example project to help define and extension 'API' for adding things to hydra editor. Also tagging @ritchse to weigh in on this.

Some basic thoughts:

1. extension is only activated when user selects it from a menu
2. registry of extensions in a separate hydra repo, can be called from editor
3. main editor is very lightweight, but extensions can add complex functions and UI elements

Basic specs

UI elements:

• extensions receive a dom element from the toolbar, and can add an icon to the toolbar
• recieve a dom element "panel" to attach additional ui elements when activated

Events:

• Extensions can listen to events from the editor, such as 'eval'
• Extensions have access to editor view state
• Extensions have access to calling certain functions from hydra editor, such as setting editor text and calling "eval"

Questions:

• a lot of this is similar in goals to CodeMirror6 extension API....since hydra will transition to CM6, to what extent should we just use the CM6 extension API vs. inventing a new one?
• how to dynamically load from the browser? similar to 'loadScript' function but more complex