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/file.markdown

@@ -25,6 +25,11 @@ automatically closed.
 
 Deletes the file at `path`.
 
+### File.**exists**(path)
+
+Whether a regular file exists at `path`. This returns `false` for directories
+or other special file system entities.
+
 ### File.**open**(path, fn)
 
 Opens the file at `path` for reading and passes it to `fn`. After the function