Rework plugin structure for autocompletion

[Twixes]

The current way of building plugins offers no autocompletion, even though it's based on TypeScript. That's because we're exporting relevant functions at the top level. Instead, a plugin should export default an object implementing interface Plugin – this would offer much better guidance to plugin builders.

[mariusandra]

I'm not sure I agree with this. The consensus between JS folks seems to be to move away from default (nameless) exports.

In addition, unless you build plugins with a TS/babel step, you can't use import/export at all. Thus this will not help people using the inline plugin editor.

Can't the same be achieved with some declarations.d.ts file?

[Twixes]

Hm, will investigate how a declarations file would work (some time).