Add 55 date style to recognized styles list

[Cellane]

I encountered today a workbook file that was impossible to read with xlsxir due to containing the number format 55.

After consulting https://learn.microsoft.com/en-us/dotnet/api/documentformat.openxml.spreadsheet.numberingformat?view=openxml-2.8.1, it seems that the format represents a date, and is related to Japanese dates only, which perfectly matches the expectations I had from this workbook file (Japanese client; not reproducible after opening and re-saving the workbook with Microsoft Excel on my local computer).

Ref: https://hexdocs.pm/xlsxir/number_styles.html#content Ref: https://learn.microsoft.com/en-us/dotnet/api/documentformat.openxml.spreadsheet.numberingformat?view=openxml-2.8.1