Add named colors from CSS Color Module Level 4

Summary

The Swing HTML StyleSheet.stringToColor() method is updated to support CSS 4 color names and hexadecimal strings supporting alpha.

Problem

The Swing HTML StyleSheet.stringToColor() method is specified as supporting only HTML 3.2 color strings, and hex strings without alpha support. CSS 4 has many additional color strings that can't be used.

Solution

Update the implementation and specification to support CSS4 color names and hex strings. This does not advance anything else in the Swing HTML support beyond HTML 3.2

Specification

Update the method java.awt.Color javax.swing.html.StyleSheet.stringToColor(String string)

     /**
-     * Converts a color string such as "RED" or "#NNNNNN" to a Color.
-     * Note: This will only convert the HTML3.2 color strings
-     *       or a string of length 7;
-     *       otherwise, it will return null.
+     * Converts a color string such as "RED",  "rgb(r g b)", "rgb(r g b a)",
+     * "rgba(r g b a)" or "#NNN", "#NNNN", "#NNNNNN",
+     * "#NNNNNNNN" to a Color.
+     * 
+     * Note: This will only convert strings which use any of the following:
+     * 
+     *   https://www.w3.org/TR/css-color-4/#named-colors
+     *   https://www.w3.org/TR/css-color-4/#hex-notation"
+     * starting with {@code #} followed by 3, 4, 6, or 8 hexadecimal digits,
+     *   https://www.w3.org/TR/css-color-4/#rgb-functions" `rgb()` and `rgba()`
+     * functions
+     *
+     * as specified by the CSS Color Module Level 4 https://www.w3.org/TR/css-color-4
+     * Otherwise, it will return null.
+     *
+     * This method is case-insensitive.
+     *
+     * The following code defines instances of the same color :
+     * {@snippet lang="java" :
+     *   import java.awt.Color;
+     *   import javax.swing.text.html.StyleSheet;
+     *   StyleSheet styleSheet = new StyleSheet();
+     *   // An opaque lightseagreen
+     *   Color color0 = styleSheet.stringToColor("Lightseagreen");
+     *   Color color1 = styleSheet.stringToColor("#20b2aa");
+     * }
      *
-     * @param string color string such as "RED" or "#NNNNNN"
+     * @param string color, string such as "RED" or "rgb(r g b)",  "rgba(r g b a)"
+     * or "#NNN", "#NNNN", "#NNNNNN", "#NNNNNNNN".
      * @return the color
      */
     public Color stringToColor(String string)