feat: add count method to Sequence class with iteration-based element tally

Implement a `count` method on the `Sequence` class that iterates over all elements and returns the total number. The method uses a simple for-loop to increment a counter for each element, providing a generic way to determine sequence length without requiring a separate length property. Includes a test sequence that yields values 1 through 10 to verify the count returns 10.

test/sequence/count.wren

@@ -0,0 +1,11 @@
+class TestSequence is Sequence {
+  iterate(iterator) {
+    if (iterator == null) return 1
+    if (iterator == 10) return false
+    return iterator + 1
+  }
+
+  iteratorValue(iterator) { iterator }
+}
+
+IO.print((new TestSequence).count) // expect: 10