Added a password rule engine

Bitvault/DigitRule.cs

@@ -0,0 +1,13 @@
+using System.Text.RegularExpressions;
+
+namespace Bitvault;
+
+public partial class DigitRule : 
+    IPasswordRule
+{
+    public int CalculateScore(string password) =>
+        Regex().IsMatch(password) ? 2 : 0;
+
+    [GeneratedRegex(@"\d")]
+    private static partial Regex Regex();
+}