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.

src/vm/wren_core.c

@@ -62,6 +62,16 @@ static const char* libSource =
 "    return result\n"
 "  }\n"
 "\n"
+"  count(f) {\n"
+"    var result = 0\n"
+"    for (element in this) {\n"
+"      if (f.call(element)) {\n"
+"        result = result + 1\n"
+"      }\n"
+"    }\n"
+"    return result\n"
+"  }\n"
+"\n"
 "  map(f) {\n"
 "    var result = new List\n"
 "    for (element in this) {\n"