Added some additional helpers for both Range and Position (#410)

* Added some additional helpers for both Range and Position to make usage easier
* Added TestContent helper for helping to test source code

Author: david-driscoll

package-lock.json

@@ -0,0 +1,13 @@
+namespace OmniSharp.Extensions.LanguageServer.Protocol.Models
+{
+    public partial class Position
+    {
+        /// <summary>
+        /// Derive a new position from this position.
+        /// </summary>
+        public Position Delta(int deltaLine = 0, int deltaCharacter = 0)
+        {
+            return new Position(Line + deltaLine, Character + deltaCharacter);
+        }
+    }
+}

src/Protocol/Models/Position.Helpers.cs

@@ -5,7 +5,7 @@
 namespace OmniSharp.Extensions.LanguageServer.Protocol.Models
 {
     [DebuggerDisplay("{" + nameof(DebuggerDisplay) + ",nq}")]
-    public class Position : IEquatable<Position>, IComparable<Position>, IComparable
+    public partial  class Position : IEquatable<Position>, IComparable<Position>, IComparable
     {
         public Position()
         {