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/string/iterate.wren

@@ -0,0 +1,16 @@
+var s = "abçd"
+IO.print(s.iterate(null)) // expect: 0
+IO.print(s.iterate(0)) // expect: 1
+IO.print(s.iterate(1)) // expect: 2
+// Skip 3 because that's the middle of the ç sequence.
+IO.print(s.iterate(2)) // expect: 4
+// Iterating from the middle of a UTF-8 sequence goes to the next one.
+IO.print(s.iterate(3)) // expect: 4
+IO.print(s.iterate(4)) // expect: false
+
+// Out of bounds.
+IO.print(s.iterate(123)) // expect: false
+IO.print(s.iterate(-1)) // expect: false
+
+// Nothing to iterate in an empty string.
+IO.print("".iterate(null)) // expect: false