Add interactive prompts for selecting values

* Adds SelectionPrompt
* Adds MultiSelectionPrompt

Closes #210

docs/input/prompts/index.cshtml

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+Title: Prompts
+Order: 5
+---
+
+<h1>Sections</h1>
+
+<ul>
+@foreach (IDocument child in OutputPages.GetChildrenOf(Document))
+{
+  <li>@Html.DocumentLink(child)</li>
+}
+</ul>

docs/input/prompts/multiselection.md

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+Title: Multi Selection
+Order: 3
+---
+
+The `MultiSelectionPrompt` can be used when you want the user to select
+one or many items from a provided list.
+
+<img src="../assets/images/multiselection.gif" style="width: 100%;" />
+
+# Usage
+
+```csharp
+// Ask for the user's favorite fruits
+var fruits = AnsiConsole.Prompt(
+    new MultiSelectionPrompt<string>()
+        .Title("What are your [green]favorite fruits[/]?")
+        .NotRequired() // Not required to have a favorite fruit
+        .PageSize(10)
+        .AddChoice("Apple")
+        .AddChoices(new[] {
+            "Apricot", "Avocado", 
+            "Banana", "Blackcurrant", "Blueberry",
+            "Cherry", "Cloudberry", "Cocunut",
+        }));
+
+// Write the selected fruits to the terminal
+foreach (string fruit in fruits) 
+{
+    AnsiConsole.WriteLine(fruit);
+}
+```

docs/input/prompts/selection.md

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+Title: Selection
+Order: 1
+---
+
+The `SelectionPrompt` can be used when you want the user to select
+a single item from a provided list.
+
+<img src="../assets/images/selection.gif" style="width: 100%;" />
+
+# Usage
+
+```csharp
+// Ask for the user's favorite fruit
+var fruit = AnsiConsole.Prompt(
+    new SelectionPrompt<string>()
+        .Title("What's your [green]favorite fruit[/]?")
+        .PageSize(10)
+        .AddChoice("Apple")
+        .AddChoices(new[] {
+            "Apricot", "Avocado", 
+            "Banana", "Blackcurrant", "Blueberry",
+            "Cherry", "Cloudberry", "Cocunut",
+        }));
+
+// Echo the fruit back to the terminal
+AnsiConsole.WriteLine($"I agree. {fruit} is tasty!");
+```

docs/input/prompts/text.md

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+Title: Text
+Order: 0
+RedirectFrom: prompt
+---
+
+Sometimes you want to get some input from the user, and for this
+you can use the `Prompt<TResult>`.
+
+# Confirmation
+
+```csharp
+if (!AnsiConsole.Confirm("Run example?"))
+{
+    return;
+}
+```
+
+```text
+Run example? [y/n] (y): _
+```
+
+# Simple
+
+```csharp
+// Ask for the user's name
+string name = AnsiConsole.Ask<string>("What's your [green]name[/]?");
+
+// Ask for the user's age
+int age = AnsiConsole.Ask<int>("What's your [green]age[/]?");
+```
+
+```text
+What's your name? Patrik
+What's your age? 37
+```
+
+# Choices
+
+```csharp
+var fruit = AnsiConsole.Prompt(
+    new TextPrompt<string>("What's your [green]favorite fruit[/]?")
+        .InvalidChoiceMessage("[red]That's not a valid fruit[/]")
+        .DefaultValue("Orange")
+        .AddChoice("Apple")
+        .AddChoice("Banana")
+        .AddChoice("Orange"));
+```
+
+```text
+What's your favorite fruit? [Apple/Banana/Orange] (Orange): _
+```
+
+# Validation
+
+```csharp
+var age = AnsiConsole.Prompt(
+    new TextPrompt<int>("What's the secret number?")
+        .Validate(age =>
+        {
+            return age switch
+            {
+                < 99 => ValidationResult.Error("[red]Too low[/]"),
+                > 99 => ValidationResult.Error("[red]Too high[/]"),
+                _ => ValidationResult.Success(),
+            };
+        }));
+```
+
+```text
+What's the secret number? 32
+Too low
+What's the secret number? 102
+Too high
+What's the secret number? _
+```
+
+# Secrets
+
+```csharp
+var password = AnsiConsole.Prompt(
+    new TextPrompt<string>("Enter [green]password[/]")
+        .PromptStyle("red")
+        .Secret());
+```
+
+```text
+Enter password: ************_
+```
+
+# Optional
+
+```csharp
+var color = AnsiConsole.Prompt(
+    new TextPrompt<string>("[grey][[Optional]][/] [green]Favorite color[/]?")
+        .AllowEmpty());
+```
+
+```text
+[Optional] Favorite color? _
+```