`drag_state` needs some types

[joshua-dean]

When index.js is moved to Typescript (#180), the drag_state needs some types.
See here:

ulabel/src/index.js

Lines 1033 to 1042 in fe138c8

	"annotation": {
	"mouse_start": null, // Screen coordinates where the current mouse drag started
	"offset_start": null, // Scroll values where the current mouse drag started
	"zoom_val_start": null // zoom_val when the dragging interaction started
	},
	"brush": {
	"mouse_start": null, // Screen coordinates where the current mouse drag started
	"offset_start": null, // Scroll values where the current mouse drag started
	"zoom_val_start": null // zoom_val when the dragging interaction started
	},

The same object declaration exists 7 times.