Added test case for syntax highlighting rule for DS digits

@Benglin PTAL
Now I see how you're using ```DigitRule``` static property, ultimately you're interested in making sure the ```Regex``` matches according to plan. In which case why don't we have a static method on ```CodeBlockEditor``` that makes you a ```Regex``` for use here? The ```CodeBlockEditor``` can make use of the same method for its internal new rule creation.