Update emoji support

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

docs/input/appendix/emojis.md

@@ -0,0 +1,38 @@
+Title: Emojis
+Order: 3
+---
+
+Please note that what emojis that can be used is completely up to 
+the operating system and/or terminal you're using, and no guarantees
+can be made of how it will look. Calculating the width of emojis
+is also not an exact science in many ways, so milage might vary when
+used in tables, panels or grids.
+
+To ensure best compatibility, consider only using emojis introduced
+before Unicode 13.0 that belongs in the `Emoji_Presentation` category
+in the official emoji list at 
+https://www.unicode.org/Public/UCD/latest/ucd/emoji/emoji-data.txt
+
+# Usage
+
+```csharp
+// Markup
+AnsiConsole.MarkupLine("Hello :globe_showing_europe_africa:!");
+
+// Constant
+var hello = "Hello " + Emoji.Known.GlobeShowingEuropeAfrica;
+```
+
+# Replacing emojis in text
+
+```csharp
+var phrase = "Mmmm :birthday_cake:";
+var rendered = Emoji.Replace(phrase);
+```
+
+# Emojis
+
+_The images in the table below might not render correctly in your 
+browser for the same reasons mentioned in the `Compatibility` section._
+
+<?# EmojiTable /?>