More file IO!

Can now create, delete, and write to files.

test/io/file/write_bytes.wren

@@ -0,0 +1,12 @@
+import "io" for File
+
+File.create("file.temp") {|file|
+  // Appends.
+  file.writeBytes("one")
+  file.writeBytes("two")
+  file.writeBytes("three")
+}
+
+System.print(File.read("file.temp")) // expect: onetwothree
+
+File.delete("file.temp")