feat: add Stat.isFile, Stat.isDirectory, File.exists, and Directory.exists methods

Add four new file system checking functions: Stat.isFile and Stat.isDirectory for querying entry type from a Stat object, and File.exists and Directory.exists static methods that return booleans by attempting to stat the path and checking the appropriate type. Also bump MAX_METHODS_PER_CLASS from 12 to 14 to accommodate the new Stat foreign methods, and clean up unused Scheduler imports in existing stat tests.

test/io/directory/exists_path_not_string.wren

@@ -0,0 +1,3 @@
+import "io" for Directory
+
+Directory.exists(123) // expect runtime error: Path must be a string.