Programs now built using IProgram interface

- The old App abstract class has been scrapped in favor of the IProgram interface in the Tomas.Interface library.
- A new terminal emulator was created when not on Windows.
- In both OS and terminal, a single dictionary has the commands and classes that implement IProgram.
- Start() now returns a boolean that behaves similar to C's main in returning an integer.
- A new while loop in both terminal and kernel handles the execution of programs.

src/Tomas.Interface/Shell/IProgram.cs

@@ -0,0 +1,7 @@
+namespace Tomas.Interface.Shell
+{
+    public interface IProgram
+    {
+        bool Start();
+    }
+}

src/Tomas.Interface/Shell/IShell.cs

@@ -0,0 +1,7 @@
+namespace Tomas.Interface.Shell
+{
+    public interface IShell
+    {
+        string ReadLine { get; }
+    }
+}

src/Tomas.Interface/Tomas.Interface.csproj

@@ -0,0 +1,7 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>netstandard2.0</TargetFramework>
+    </PropertyGroup>
+
+</Project>