feat: make String iterable over code points with iterate/iteratorValue primitives

Add String as a subclass of Sequence in core.wren and implement the iterator protocol for strings in wren_core.c. The string_iterate native advances through UTF-8 code point boundaries, while string_iteratorValue extracts the full code point at a given byte index. Includes comprehensive test coverage for iteration, edge cases (empty strings, mid-sequence positions), and error handling for invalid iterator types and out-of-bounds indices. Also updates documentation for String, List, and Sequence classes with iterator protocol details and usage examples.

test/list/iterate.wren

@@ -4,6 +4,9 @@ IO.print(a.iterate(0)) // expect: 1
 IO.print(a.iterate(1)) // expect: 2
 IO.print(a.iterate(2)) // expect: 3
 IO.print(a.iterate(3)) // expect: false
+
+// Out of bounds.
+IO.print(a.iterate(123)) // expect: false
 IO.print(a.iterate(-1)) // expect: false
 
 // Nothing to iterate in an empty list.