src/vs/base/common/actions.ts/Separator/join
class Separator:	public static join(...actionLists: readonly IAction[][]) {
		let out: IAction[] = [];
		for (const list of actionLists) {
			if (!list.length) {
				// skip
			} else if (out.length) {
				out = [...out, new Separator(), ...list];
			} else {
				out = list;
			}
		}

		return out;
	}