AK: Added contains_in_range to Vector

Vector::contains_in_range() allows the search of an element in a given
range on a vector object.
Also added testcases for the new Vector method.

AK/Vector.h

@@ -150,6 +150,19 @@ public:
         return false;
     }
 
+    bool contains_in_range(const T& value, const size_t start, const size_t end) const
+    {
+        VERIFY(start <= end);
+        VERIFY(end < size());
+        for (size_t i = start; i <= end; ++i) {
+            if (Traits<T>::equals(at(i), value))
+                return true;
+        }
+        return false;
+    }
+
+    // NOTE: Vector::is_null() exists for the benefit of String::copy().
+    bool is_null() const { return false; }
     bool is_empty() const { return size() == 0; }
     ALWAYS_INLINE size_t size() const { return m_size; }
     size_t capacity() const { return m_capacity; }

Tests/AK/TestVector.cpp

@@ -399,3 +399,43 @@ TEST_CASE(should_find_index)
     EXPECT_EQ(4u, v.find_first_index(0).value());
     EXPECT(!v.find_first_index(42).has_value());
 }
+
+TEST_CASE(should_contain_start)
+{
+    // Tests whether value is found if at the start of the range.
+    Vector<int> v { 1, 2, 3, 4, 5 };
+
+    EXPECT(v.contains_in_range(1, 0, 4));
+}
+
+TEST_CASE(should_contain_end)
+{
+    // Tests whether value is found if at the end of the range.
+    Vector<int> v { 1, 2, 3, 4, 5 };
+
+    EXPECT(v.contains_in_range(5, 0, 4));
+}
+
+TEST_CASE(should_contain_range)
+{
+    // Tests whether value is found within a range.
+    Vector<int> v { 1, 2, 3, 4, 5 };
+
+    EXPECT(v.contains_in_range(3, 0, 4));
+}
+
+TEST_CASE(should_not_contain_not_present)
+{
+    // Tests whether a value that is not present is not found, as expected.
+    Vector<int> v { 1, 2, 3, 4, 5 };
+
+    EXPECT(!v.contains_in_range(6, 0, 4));
+}
+
+TEST_CASE(should_not_contain_present_not_in_range)
+{
+    // Tests whether a value that is present, but not in range, is not found.
+    Vector<int> v { 1, 2, 3, 4, 5 };
+
+    EXPECT(!v.contains_in_range(2, 2, 4));
+}