feat: add Sequence.count(predicate) method and update documentation

Implement a new `count(f)` method on the Sequence class that accepts a predicate function and returns the number of elements for which the predicate evaluates to true. The implementation iterates over the sequence, calling `f.call(element)` on each element and incrementing a counter when the result is truthy.

Also update the documentation for both Sequence and List classes: add full documentation for the new `count(predicate)` method on Sequence, and change the terminology in List docs from "items" to "elements" for consistency. Add three test files covering the basic predicate behavior, non-boolean return values from the predicate, and the error case when a non-function argument is passed.

builtin/core.wren

@@ -16,6 +16,16 @@ class Sequence {
     return result
   }
 
+  count(f) {
+    var result = 0
+    for (element in this) {
+      if (f.call(element)) {
+        result = result + 1
+      }
+    }
+    return result
+  }
+
   map(f) {
     var result = new List
     for (element in this) {