Start sketching in support for reading from stdin.

test/io/stdin/aborts_on_eof.wren

@@ -0,0 +1,9 @@
+import "io" for Stdin
+
+Stdin.readLine() // stdin: one line
+
+var error = Fiber.new {
+  Stdin.readLine()
+}.try()
+
+System.print(error) // expect: Stdin was closed.