Add ReC.Domain project to solution

A new project, `ReC.Domain`, has been added to the solution.
The project is included in `ReC.sln` with build configurations
for `Debug|Any CPU` and `Release|Any CPU`. It is nested under
the `src` folder in the solution structure.

The `ReC.Domain.csproj` file targets .NET 8.0, uses the
`Microsoft.NET.Sdk` SDK, and enables modern C# features such
as implicit `using` directives and nullable reference types.

src/ReC.Domain/ReC.Domain.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>