More file IO!

Can now create, delete, and write to files.

test/io/file/create.wren

@@ -0,0 +1,22 @@
+import "io" for File
+
+// Create a new file.
+var file = File.create("file.temp")
+System.print(file.isOpen) // expect: true
+System.print(file.size) // expect: 0
+
+file.writeBytes("stuff")
+file.close()
+
+System.print(File.size("file.temp")) // expect: 5
+
+// Overwrite a file.
+file = File.create("file.temp")
+
+// Truncates.
+System.print(file.size) // expect: 0
+file.close()
+
+File.delete("file.temp")
+
+// TODO: Test it cannot be read from.