feat: add Directory.list() method to io module for reading directory entries

Implement a new `Directory` class with a static `list(path)` method that returns an array of filenames from the specified directory. The implementation uses a foreign function `list_(path, fiber)` that performs the actual filesystem read via libuv, collects all directory entries into a zero-byte-terminated buffer, and then splits that buffer into individual strings on the Wren side. Includes test cases for normal directory listing, listing a file path (runtime error), listing a nonexistent path (runtime error), and passing a wrong argument type (runtime error). Also bumps `MAX_CLASSES_PER_MODULE` from 3 to 4 to accommodate the new class.

test/io/directory/list_nonexistent.wren

@@ -0,0 +1,3 @@
+import "io" for Directory
+
+Directory.list("nonexistent") // expect runtime error: no such file or directory