More file system checking functions:

- Stat#isFile
- Stat#isDirectory
- Directory.exists()
- File.exists()

test/io/file/exists.wren

@@ -0,0 +1,9 @@
+import "io" for File
+
+System.print(File.exists("test/io/file/file.txt")) // expect: true
+System.print(File.exists("nonexistent")) // expect: false
+
+// Directories are not files.
+System.print(File.exists("test/io/file")) // expect: false
+
+// TODO: Symlinks.