Fixes #2617 - Support for Nerd icons for FileDialog (#2613)

* Add explicit file name unicode mappings

* Added nerd file extensions

* Add folder icon

* Added tests and attribution

* Fix tree view not using System.IO.Abstractions

* Make FileDialog tree use IconGetter

* Added context to IconGetter

* Make Nerd a config setting

* Fix typo in attribution

* tidy up

* Change open folder check to null or whitespace

* Update class diagram to show new classes

* Rename configuration property NerdFonts and set it to ThemeScope

* Move NerdFonts setting to ConsoleDriver

* Move NerdFonts setting to NerdFonts class and rename Enable

---------

Co-authored-by: Tig <tig@users.noreply.github.com>

UnitTests/FileServices/NerdFontsTests.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.IO;
+using System.IO.Abstractions.TestingHelpers;
+using System.Linq;
+using Terminal.Gui;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Terminal.Gui.FileServicesTests {
+
+    public class NerdFontTests
+    {
+        [Fact]
+        public void TestAllFilenamesMapToKnownGlyphs()
+        {
+            var f = new NerdFonts();
+            foreach(var k in f.FilenameToIcon)
+            {
+                Assert.Contains(k.Value, f.Glyphs.Keys);
+            }
+        }
+
+
+        [Fact]
+        public void TestAllExtensionsMapToKnownGlyphs()
+        {
+            var f = new NerdFonts();
+            foreach(var k in f.ExtensionToIcon)
+            {
+                Assert.Contains(k.Value, f.Glyphs.Keys);
+            }
+        }
+    }
+
+}