Add initial project structure with Result type and extension methods

Rehlert.RoP/Extensions/FallbackExtensions.cs

@@ -0,0 +1,28 @@
+namespace Rehlert.RoP.Extensions;
+
+public static class FallbackExtensions
+{
+    public static TIn FallbackWith<TIn, TErr>(this Result<TIn, TErr> value, TIn defaultValue)
+    {
+        var result = value.Match(
+            ok => ok.Value,
+            _ => defaultValue
+        );
+
+        return result;
+    }
+
+    public static async Task<TIn> FallbackWith<TIn, TErr>(
+        this Task<Result<TIn, TErr>> value,
+        TIn defaultValue
+    )
+    {
+        var valRes = await value;
+        var result = valRes.Match(
+            ok => ok.Value,
+            _ => defaultValue
+        );
+
+        return result;
+    }
+}