recognize bigint as stringifiable

[bug-brain]

This would be a nice feature to prevent some explicit .toString()´ calls before passing an object to stringify`.

[sindresorhus]

I'm willing to merge this, but you need to fix the lint issues.

[bug-brain]

Going for xo@0.56.0 would require (edit: shows up as red) using String#replaceAll() instead of String#replace(). For the two usages this would not be breaking but I prefer changing less for this feature because it is technically only a typing issue.