feat: add type validation for IO.read prompt and stdin support in test runner

Add string type check for IO.read() prompt argument with Fiber.abort on non-string input. Implement stdin piping in test runner via // stdin: comments, allowing tests to provide input lines. Update stack trace filtering to omit built-in library frames with empty source paths. Add io/read.wren and io/read_arg_not_string.wren test cases.

include/wren.h

@@ -85,6 +85,10 @@ WrenVM* wrenNewVM(WrenConfiguration* configuration);
 void wrenFreeVM(WrenVM* vm);
 
 // Runs [source], a string of Wren source code in a new fiber in [vm].
+//
+// [sourcePath] is a string describing where [source] was located, for use in
+// debugging stack traces. It must not be `null`. If an empty string, it will
+// not be shown in a stack trace.
 WrenInterpretResult wrenInterpret(WrenVM* vm, const char* sourcePath,
                                   const char* source);