Add "io" module with beginnings of a File class.

test/io/file/size.wren

@@ -0,0 +1,18 @@
+import "io" for File
+import "scheduler" for Scheduler
+
+System.print(File.size("test/io/file/size.wren")) // expect: 401
+
+// Runs asynchronously.
+Scheduler.add {
+  System.print("async")
+}
+
+System.print(File.size("test/io/file/size.wren"))
+// expect: async
+// expect: 401
+
+var error = Fiber.new {
+  System.print(File.size("nonexistent"))
+}.try()
+System.print(error) // expect: no such file or directory