feat: add Sequence.count(predicate) method and update docs/tests

Implement a new `count(f)` method on the Sequence class that iterates over elements and returns the number of items for which the given predicate function evaluates to true. Update the core library source, sequence documentation, and add three new test files covering normal usage, non-boolean return values, and non-function argument error handling. Also fix minor terminology in list docs (items → elements).

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"