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/dir/a.txt

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+this is a text file

test/io/directory/dir/b.txt

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+this is a text file

test/io/directory/dir/c.txt

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+this is a text file

test/io/directory/list.wren

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+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]

test/io/directory/list_file.wren

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+import "io" for Directory
+
+var entries = Directory.list("test/io/directory/dir/a.txt") // expect runtime error: not a directory

test/io/directory/list_nonexistent.wren

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+import "io" for Directory
+
+Directory.list("nonexistent") // expect runtime error: no such file or directory

test/io/directory/list_wrong_arg_type.wren

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+import "io" for Directory
+
+Directory.list(123) // expect runtime error: Path must be a string.