Add example infrastructure

* Add "Shared" projects for all examples
* Update "Colors" example with better TrueColor demo
* Use same namespace for all examples

examples/Shared/ColorBox.cs

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+using System;
+using System.Collections.Generic;
+using Spectre.Console.Rendering;
+
+namespace Spectre.Console.Examples
+{
+    public sealed class ColorBox : Renderable
+    {
+        private readonly int _height;
+        private int? _width;
+
+        public ColorBox(int height)
+        {
+            _height = height;
+        }
+
+        public ColorBox(int width, int height)
+            : this(height)
+        {
+            _width = width;
+        }
+
+        protected override Measurement Measure(RenderContext context, int maxWidth)
+        {
+            return new Measurement(1, GetWidth(maxWidth));
+        }
+
+        protected override IEnumerable<Segment> Render(RenderContext context, int maxWidth)
+        {
+            maxWidth = GetWidth(maxWidth);
+
+            for (var y = 0; y < _height; y++)
+            {
+                for (var x = 0; x < maxWidth; x++)
+                {
+                    var h = x / (float)maxWidth;
+                    var l = 0.1f + ((y / (float)_height) * 0.7f);
+                    var (r1, g1, b1) = ColorFromHSL(h, l, 1.0f);
+                    var (r2, g2, b2) = ColorFromHSL(h, l + (0.7f / 10), 1.0f);
+
+                    var background = new Color((byte)(r1 * 255), (byte)(g1 * 255), (byte)(b1 * 255));
+                    var foreground = new Color((byte)(r2 * 255), (byte)(g2 * 255), (byte)(b2 * 255));
+
+                    yield return new Segment("▄", new Style(foreground, background));
+                }
+
+                yield return Segment.LineBreak;
+            }
+        }
+
+        private int GetWidth(int maxWidth)
+        {
+            var width = maxWidth;
+            if (_width != null)
+            {
+                width = Math.Min(_width.Value, width);
+            }
+
+            return width;
+        }
+
+        private static (float, float, float) ColorFromHSL(double h, double l, double s)
+        {
+            double r = 0, g = 0, b = 0;
+            if (l != 0)
+            {
+                if (s == 0)
+                {
+                    r = g = b = l;
+                }
+                else
+                {
+                    double temp2;
+                    if (l < 0.5)
+                    {
+                        temp2 = l * (1.0 + s);
+                    }
+                    else
+                    {
+                        temp2 = l + s - (l * s);
+                    }
+
+                    var temp1 = 2.0 * l - temp2;
+
+                    r = GetColorComponent(temp1, temp2, h + 1.0 / 3.0);
+                    g = GetColorComponent(temp1, temp2, h);
+                    b = GetColorComponent(temp1, temp2, h - 1.0 / 3.0);
+                }
+            }
+
+            return ((float)r, (float)g, (float)b);
+
+        }
+
+        private static double GetColorComponent(double temp1, double temp2, double temp3)
+        {
+            if (temp3 < 0.0)
+            {
+                temp3 += 1.0;
+            }
+            else if (temp3 > 1.0)
+            {
+                temp3 -= 1.0;
+            }
+
+            if (temp3 < 1.0 / 6.0)
+            {
+                return temp1 + (temp2 - temp1) * 6.0 * temp3;
+            }
+            else if (temp3 < 0.5)
+            {
+                return temp2;
+            }
+            else if (temp3 < 2.0 / 3.0)
+            {
+                return temp1 + ((temp2 - temp1) * ((2.0 / 3.0) - temp3) * 6.0);
+            }
+            else
+            {
+                return temp1;
+            }
+        }
+    }
+}

examples/Shared/Extensions/ColorExtensions.cs

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+namespace Spectre.Console.Examples
+{
+    public static class ColorExtensions
+    {
+        public static Color GetInvertedColor(this Color color)
+        {
+            return GetLuminance(color) < 140 ? Color.White : Color.Black;
+        }
+
+        private static float GetLuminance(this Color color)
+        {
+            return (float)((0.2126 * color.R) + (0.7152 * color.G) + (0.0722 * color.B));
+        }
+    }
+}

examples/Shared/Shared.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net5.0</TargetFramework>
+    <ExampleVisible>false</ExampleVisible>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Spectre.Console\Spectre.Console.csproj" />
+  </ItemGroup>
+
+</Project>