Add gatherer for scan

Author: ZahidFKhan

src/test/java/com/github/streams/learn/b_advanced/gatherers/D_Inbuilt_Gatherers.md

@@ -1,5 +1,5 @@
 ## Inbuilt Gatherers?
 
 1. Scan
-2. 
+2. FixedWindow
 3. 

src/test/java/com/github/streams/practice/gatherers/ScanTest.java

@@ -0,0 +1,25 @@
+package com.github.streams.practice.gatherers;
+
+import java.util.List;
+import java.util.stream.Gatherers;
+import java.util.stream.Stream;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.condition.EnabledOnJre;
+import org.junit.jupiter.api.condition.JRE;
+
+
+class ScanTest {
+  @Test
+  @EnabledOnJre(JRE.JAVA_25)
+  void scanTest() {
+    final List<String> actual =
+        Stream.of(1, 2, 3, 4, 5, 6, 7, 8, 9)
+            .gather(Gatherers.scan(() -> "", (string, number) -> string + number))
+            .toList();
+
+    final List<String> expected =
+        List.of("1", "12", "123", "1234", "12345", "123456", "1234567", "12345678", "123456789");
+    Assertions.assertEquals(expected, actual);
+  }
+}