Most regular expression engines don't accept the `{,n}` syntax, but some other do it (namely Python's `re` library). This introduces a new parser configuration option that enables the `{,n}` syntax.

… in test case

Now `RepetitionCountUnclosed` error has precedence over `RepetitionCountDecimalEmpty`, even if the `{,n}` syntax is not accepted. I think this is even desirable, as `RepetitionCountUnclosed` describes the situation better when an unclosed `{` is found in the regex.