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.

builtin/core.wren

@@ -1,4 +1,12 @@
 class Sequence {
+  count {
+    var result = 0
+    for (element in this) {
+      result = result + 1
+    }
+    return result
+  }
+
   map(f) {
     var result = new List
     for (element in this) {