Update emoji support

* Add constants for emojis
* Move emoji shortcode rendering to Markup
* Add documentation
* Add example
* Add tests

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

@@ -0,0 +1,44 @@
+using Shouldly;
+using Xunit;
+
+namespace Spectre.Console.Tests.Unit
+{
+    public sealed class EmojiTests
+    {
+        [Fact]
+        public void Should_Substitute_Emoji_Shortcodes_In_Markdown()
+        {
+            // Given
+            var console = new TestableAnsiConsole(ColorSystem.Standard, AnsiSupport.Yes);
+
+            // When
+            console.Markup("Hello :globe_showing_europe_africa:!");
+
+            // Then
+            console.Output.ShouldBe("Hello 🌍!");
+        }
+
+        [Fact]
+        public void Should_Contain_Predefined_Emojis()
+        {
+            // Given, When
+            const string result = "Hello " + Emoji.Known.GlobeShowingEuropeAfrica + "!";
+
+            // Then
+            result.ShouldBe("Hello 🌍!");
+        }
+
+        public sealed class TheReplaceMethod
+        {
+            [Fact]
+            public void Should_Replace_Emojis_In_Text()
+            {
+                // Given, When
+                var result = Emoji.Replace("Hello :globe_showing_europe_africa:!");
+
+                // Then
+                result.ShouldBe("Hello 🌍!");
+            }
+        }
+    }
+}