Using vis.DataSet _idProp instead of _fieldId

[jasonex7]

I see that vis.DataSet defines _idProp with fieldId configuration option, but vis.Timeline uses _fieldId which is always undefined. vis.Timeline should use _idProp too.

vis.DataSet definition of _idProp:
https://github.com/visjs/vis-data/blob/3e86bc0c55425476b803412e9cea207f0e075a67/src/data-set.ts#L173

Example of vis.Timeline using _fieldId:

vis-timeline/lib/timeline/component/ItemSet.js

Lines 1604 to 1605 in 6a6899a

	const id = util.randomUUID();
	itemData[this.itemsData._fieldId] = id;