Fix BigRational#format

[meatball133]

resolves #14062

This is a small change, but it fixes the issue and is perhaps not the most efficient solution; the main goal was to not affect the current code base too much.
In the issues, there are 2 approaches mentioned; I went ahead with this one since if a user wants the result as a float, they can still do that by converting the BigRational to a float.

But I want to present this PR more as a proposal for a solution than as a final solution (thereby, no tests are written/updated). If there is consensus to take the other approach, is that also an option.

[meatball133]

Update: I forgot that the big library can't be imported, so the type of the number can't be checked. I implemented an alternative solution, but might that not be that safe?

[HertzDevil]

You could override just this overload in src/big/big_rational.cr itself, that would avoid a rather fragile check for /.