Create Glob class (for recursively matching paths to globs)

This is a custom glob class for any filesystem to use to recurse the directory structure (rather than do a full directory walk like most of the Glob classes I saw do).  I added tests for various cases.  This will be a component of editorConfig.

Please review, @devoncarew 

Thanks!
`Glob` is defined in `filesystem.dart` but it's tests in `preferences_test.dart` - we may want to reconcile that.