feat: add Stdin class with readLine and async stdin read support in io module

test/io/stdin/read_line.wren

@@ -0,0 +1,11 @@
+import "io" for Stdin
+
+System.write("> ")
+System.print("1 " + Stdin.readLine())
+System.write("> ")
+System.print("2 " + Stdin.readLine())
+
+// stdin: first
+// stdin: second
+// expect: > 1 first
+// expect: > 2 second