Implement 3 digit hex parsing (#1708)

Author: TheMarteh

src/Spectre.Console/Color.cs

@@ -230,6 +230,13 @@ public static Color FromHex(string hex)
             hex = hex.Substring(1);
         }
 
+        // 3 digit hex codes are expanded to 6 digits
+        // by doubling each digit, conform to CSS color codes
+        if (hex.Length == 3)
+        {
+            hex = string.Concat(hex.Select(c => new string(c, 2)));
+        }
+
         var r = byte.Parse(hex.Substring(0, 2), NumberStyles.HexNumber);
         var g = byte.Parse(hex.Substring(2, 2), NumberStyles.HexNumber);
         var b = byte.Parse(hex.Substring(4, 2), NumberStyles.HexNumber);

src/Tests/Spectre.Console.Tests/Unit/ColorTests.cs

@@ -68,6 +68,23 @@ public void Should_Not_Parse_Non_Color_Try_From_Hex(string noncolor)
             color.ShouldBe(Color.Default);
         }
 
+        [Theory]
+        [InlineData("ffffff")]
+        [InlineData("#ffffff")]
+        [InlineData("fff")]
+        [InlineData("#fff")]
+        public void Should_Parse_3_Digit_Hex_Colors_From_Hex(string color)
+        {
+            // Given
+            var expected = new Color(255, 255, 255);
+
+            // When
+            var result = Color.FromHex(color);
+
+            // Then
+            result.ShouldBe(expected);
+        }
+
         [Fact]
         public void Should_Consider_Color_And_Non_Color_Equal()
         {