Implement regions.label to .region() API

[netil]

Description

regions.label option added on 3.10 (#3319), but not implemented on .regions() APIs.
Implement same way as the option provides.

chart.regions([
   {
     axis: "x",
     start: 1,
     end: 4,
     class: "region-1-4",
     label: {
     	text: "Region Text",
     	x: 5,  // position relative of the initial x coordinate
     	y: 5,  // position relative of the initial y coordinate
     	color: "red",  // color string
     	rotated: true  // make text to show in vertical or horizontal
     }
   }
]);

chart.regions.add({
   ...,
   label: { ... }
});

chart.regions([
   ...,
   { ..., label: { ... } }
]);

[netil]

it already works as expected