fix: remove redundant manual increment of loop variable in List.toString

The for-in loop in Wren already increments the loop variable automatically, so the explicit `i = i + 1` inside the loop body was causing every other element to be skipped when building the string representation of a List. This fix removes the unnecessary increment from both the corelib.wren source and the embedded string in wren_core.c.

src/wren_core.c

@@ -61,7 +61,6 @@ const char* coreLibSource =
 "    for (i in 0...this.count) {\n"
 "      if (i > 0) result = result + \", \"\n"
 "      result = result + this[i].toString\n"
-"      i = i + 1\n"
 "    }\n"
 "    result = result + \"]\"\n"
 "    return result\n"