added interface for enc and dec types

Author: Petr Sramek

src/PolylineAlgorithm/PolylineDecoder.cs

@@ -9,7 +9,7 @@ namespace PolylineAlgorithm;
 
 /// <inheritdoc cref="IPolylineDecoder{TResult}" />
 /// <param name="validator">A coordinate validator.</param>
-public class PolylineDecoder {
+public class PolylineDecoder : IPolylineDecoder {
 
     /// <inheritdoc />
     /// <exception cref="ArgumentException">Thrown when <paramref name="polyline"/> argument is null -or- empty.</exception>

src/PolylineAlgorithm/PolylineEncoder.cs

@@ -12,7 +12,7 @@ namespace PolylineAlgorithm;
 /// <summary>
 /// Performs polyline algorithm decoding and encoding
 /// </summary>
-public class PolylineEncoder {
+public class PolylineEncoder : IPolylineEncoder {
     /// <summary>
     /// Encodes coordinates to polyline representation
     /// </summary>