feat: add C/c as cross-type format specifier alias for Color

C/c aliases R/r on Color (CSS rgb output), matching the convention
where C/c outputs the natural CSS format for any color type.

Author: NathanDrake2406

src/Avalonia.Base/Media/Color.cs

@@ -476,8 +476,8 @@ internal void ToString(System.Text.StringBuilder builder)
         /// <item><c>"x"</c> — XAML hex without alpha: <c>#RRGGBB</c></item>
         /// <item><c>"H"</c> — HTML hex with alpha: <c>#RRGGBBAA</c></item>
         /// <item><c>"h"</c> — HTML hex without alpha: <c>#RRGGBB</c></item>
-        /// <item><c>"R"</c> — CSS rgb with alpha: <c>rgb(r, g, b, a)</c></item>
-        /// <item><c>"r"</c> — CSS rgb without alpha: <c>rgb(r, g, b)</c></item>
+        /// <item><c>"R"</c> or <c>"C"</c> — CSS rgb with alpha: <c>rgb(r, g, b, a)</c></item>
+        /// <item><c>"r"</c> or <c>"c"</c> — CSS rgb without alpha: <c>rgb(r, g, b)</c></item>
         /// <item><c>"P"</c> — CSS rgb percent with alpha: <c>rgb(r%, g%, b%, a%)</c></item>
         /// <item><c>"p"</c> — CSS rgb percent without alpha: <c>rgb(r%, g%, b%)</c></item>
         /// </list>
@@ -498,8 +498,8 @@ public string ToString(string? format, IFormatProvider? formatProvider)
                 "x" => $"#{R:X2}{G:X2}{B:X2}",
                 "H" => $"#{R:X2}{G:X2}{B:X2}{A:X2}",
                 "h" => $"#{R:X2}{G:X2}{B:X2}",
-                "R" => string.Format(CultureInfo.InvariantCulture, "rgb({0}, {1}, {2}, {3:F2})", R, G, B, A * byteToDouble),
-                "r" => string.Format(CultureInfo.InvariantCulture, "rgb({0}, {1}, {2})", R, G, B),
+                "R" or "C" => string.Format(CultureInfo.InvariantCulture, "rgb({0}, {1}, {2}, {3:F2})", R, G, B, A * byteToDouble),
+                "r" or "c" => string.Format(CultureInfo.InvariantCulture, "rgb({0}, {1}, {2})", R, G, B),
                 "P" => FormatCssRgbPercentWithAlpha(),
                 "p" => FormatCssRgbPercentWithoutAlpha(),
                 _ => throw new FormatException($"Format string '{format}' is not supported.")

tests/Avalonia.Base.UnitTests/Media/ColorTests.cs

@@ -515,5 +515,14 @@ public void ToString_x_And_h_Produce_Same_Output()
                 color.ToString("x", null),
                 color.ToString("h", null));
         }
+
+        [Fact]
+        public void ToString_C_Is_Alias_For_R()
+        {
+            var color = new Color(0x80, 0xFF, 0x88, 0x44);
+
+            Assert.Equal(color.ToString("R", null), color.ToString("C", null));
+            Assert.Equal(color.ToString("r", null), color.ToString("c", null));
+        }
     }
 }