Add project files.

Bip21.cs

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+using System.Collections.Immutable;
+using System.Web;
+
+namespace payto;
+internal class Bip21
+{
+    // possible kyes in bip21 that can hold bolt12 offer
+    private static ImmutableArray<string> bip21_ln_keys = ["lightning", "lno", "lno1", "b12"];
+
+    /// <summary>
+    /// Try to extract bolt12 offer from bit21 string
+    /// </summary>
+    public static (bool success, string? offer) TryGetOffer(string bip21)
+    {
+        // Split the input string into the base part and the query string
+        var parts = bip21.Split('?');
+
+        if (parts.Length == 2)
+        {
+            // Parse the query string
+            var queryParams = HttpUtility.ParseQueryString(parts[1]);
+
+            // Find the first value that starts with "lno1" (case-insensitive) for any of the specified keys
+            string? lightningValue = bip21_ln_keys
+                        .Select(key => queryParams[key])
+                        .FirstOrDefault(value => value?.StartsWith("lno1", StringComparison.InvariantCultureIgnoreCase) ?? false);
+
+            //if the bip21 string contains multiple values with the same key this will be broken here, in that case ParseQueryString joins them with comma
+
+            if (lightningValue != null)
+                return (true, lightningValue);
+        }
+
+        return (false, null);
+    }
+
+}