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.

doc/site/core/string.markdown

@@ -56,6 +56,19 @@ Returns the index of the first byte matching `search` in the string or `-1` if
 
 It is a runtime error if `search` is not a string.
 
+### **iterate**(iterator), **iteratorValue**(iterator)
+
+Implements the [iterator protocol](../control-flow.html#the-iterator-protocol)
+for iterating over the *code points* in the string:
+
+    :::dart
+    var codePoints = []
+    for (c in "(ᵔᴥᵔ)") {
+      codePoints.add(c)
+    }
+
+    IO.print(codePoints) // ["(", "ᵔ", "ᴥ", "ᵔ", ")"].
+
 ### **startsWith**(prefix)
 
 Checks if the string starts with `prefix`.