Add Directory.list() to io.

test/io/directory/list.wren

@@ -0,0 +1,8 @@
+import "io" for Directory
+
+var entries = Directory.list("test/io/directory/dir")
+
+// Ignore OS-specific dot files like ".DS_Store".
+entries = entries.where {|entry| !entry.startsWith(".") }.toList
+
+System.print(entries) // expect: [a.txt, b.txt, c.txt]