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.

test/io/read_arg_not_string.wren

@@ -0,0 +1 @@
+IO.read(null) // expect runtime error: Prompt must be a string.