Plan plugin manager

[elkowar]

• plugin version negotiation (plugin API changes)
• plugin dependencies (DAG of deps? pain)

[elkowar]

We'll have a plugin manager component that plugins register with. They advertize their version, conflicts with other plugins, which plugins (at what version ranges) they depend on, and which functions they provide. Plugins may register event listeners. They may act upon the document asynchronously and query the document (those queries may of course be sync within the scope of that plugin, the plugin manager shouldn't care about that) through a defined API. The core may itself be presented as a plugin, if we can't come up with any good reasons not to do that. Thus, the plugin api should probably be transparent over plugins running in a wasm context vs. just being part of the main binary.