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.

doc/site/modules/io/directory.markdown

@@ -4,6 +4,11 @@ A directory on the file system.
 
 ## Static Methods
 
+### Directory.**exists**(path)
+
+Whether a directory exists at `path`. This returns `false` for files or other
+special file system entities.
+
 ### Directory.**list**(path)
 
 Lists the contents of the directory at `path`. Returns a sorted list of path