Complete HighchartsBarStates interface. Add Color interface (#11505)

- HighchartsBarStates interfaces didn't include `borderColor` and `color` properties, as documented in http://api.highcharts.com/highcharts/plotOptions.bar.states.hover
- A Color interface was added, since the documentation refers many times to `Color` as a type. This colors can be either a string or a gradient, as stated in http://www.highcharts.com/docs/chart-design-and-style/colors. The introduction of this type should help alleviating the errors currently present where certain "color" properties are only allowed to be strings.

highcharts/highcharts.d.ts

@@ -240,7 +240,7 @@ interface HighchartsPlotBands {
      * Border color for the plot band. Also requires borderWidth to be set.
      * @default null
      */
-    borderColor?: string | HighchartsGradient;
+    borderColor?: Color;
     /**
      * Border width for the plot band. Also requires borderColor to be set.
      * @default 0
@@ -249,7 +249,7 @@ interface HighchartsPlotBands {
     /**
      * The color of the plot band.
      */
-    color?: string | HighchartsGradient;
+    color?: Color;
     /**
      * An object defining mouse events for the plot band. Supported properties are click, mouseover, mouseout,
      * mousemove.
@@ -1309,6 +1309,11 @@ interface HighchartsGradient {
     setOpacity?(alpha: number): HighchartsGradient;
 }
 
+/**
+ * Type equivalent to the 'Color' type mentioned throughout the documentation.
+ */
+type Color = string | HighchartsGradient;
+
 interface HighchartsChartOptions3dFrame {
     /**
      * The color of the panel.
@@ -3248,11 +3253,19 @@ interface HighchartsLineStates {
 }
 
 interface HighchartsBarStates {
+    /**
+     * A specific border color for the hovered point. Defaults to inherit the normal state border color.
+     */
+    borderColor?: string | HighchartsGradient;
     /**
      * How much to brighten the point on interaction. Requires the main color to be defined in hex or rgb(a) format.
      * @default 0.1
      */
     brightness?: number;
+    /**
+     *
+     */
+    color?: string | HighchartsGradient;
     /**
      * Enable separate styles for the hovered series to visualize that the user hovers either the series itself or the
      * legend.