project: reorganize the structure of the project.

* remove dotnet-tool.json because the project does not rely on any dotnet tools.
* remove Directory.Build.props because the solution has only one project.
* move src/SourceGit to src. It's not needed to put all sources into a subfolder of src since there's only one project.

src/Models/Version.cs

@@ -0,0 +1,38 @@
+using System.Reflection;
+using System.Text.Json.Serialization;
+using System.Text.RegularExpressions;
+
+namespace SourceGit.Models
+{
+    public partial class Version
+    {
+        [JsonPropertyName("name")]
+        public string Name { get; set; }
+
+        [JsonPropertyName("tag_name")]
+        public string TagName { get; set; }
+
+        [JsonPropertyName("body")]
+        public string Body { get; set; }
+
+        [GeneratedRegex(@"^v(\d+)\.(\d+)$")]
+        private static partial Regex REG_VERSION_TAG();
+
+        public bool IsNewVersion
+        {
+            get
+            {
+                var match = REG_VERSION_TAG().Match(TagName);
+                if (!match.Success)
+                    return false;
+
+                var major = int.Parse(match.Groups[1].Value);
+                var minor = int.Parse(match.Groups[2].Value);
+                var ver = Assembly.GetExecutingAssembly().GetName().Version;
+                return ver.Major < major || (ver.Major == major && ver.Minor < minor);
+            }
+        }
+    }
+
+    public class AlreadyUpToDate { }
+}