Add Directory.list() to io.

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.