fix(schedule): ensure auto generated task IDs are always unique

Previously, we were generating task IDs based on the length of the tasks object. However, this falls
short if any tasks have been unscheduled. For example, let's say we schedule 2 tasks, resulting in
tasks with IDs of `"0"` and `"1"`. Then we unschedule task `"0"`. The length of the tasks object is
now `1`, so when we attempt to schedule the next task we will use the ID `"1"`. This will conflict
with the existing task, causing an error to be thrown.

lib/schedule.js

@@ -10,6 +10,13 @@ import { state } from './state.js'
 
 
 
+// Variables
+let nextTaskID = 0
+
+
+
+
+
 /**
  * Adds a task to the schedule, ensuring it is run on the `requestAnimationFrame` loop.
  *
@@ -23,13 +30,20 @@ export function schedule(task, options = {}) {
 		context,
 		framerate = 60,
 		isPaused = false,
-		id: taskID = String(Object.keys(state.tasks).length),
 	} = options
+	let { id: taskID } = options
 
 	if (framerate > 60) {
 		throw new RangeError(`Framerate may not be higher than 60; requested framerate is ${framerate}.`)
 	}
 
+	if (!taskID) {
+		taskID = String(nextTaskID)
+		nextTaskID += 1
+	} else if ((typeof taskID === 'number') && !Number.isNaN(taskID)) {
+		nextTaskID = taskID + 1
+	}
+
 	if (state.tasks[taskID]) {
 		throw new RangeError(`A task with the ID \`${taskID}\` already exists.`)
 	}