Codepage support

[Oakchris1955]

Windows used codepages for encoding characters before Unicode was a thing. While VFAT support Unicode for its long filenames (more specifically, little-endian UTF-16), decoding short filenames depends on the currently selected codepage. f9d2338 implements support for some basic single-byte codepages, however there are also other multi-byte codepages that I haven't add support for yet.

One option is to support the most commonly-used ones, such as the Japanese one, and also add a field under the Codepage struct for unsupported codepages, where the user will be able to provide their own.

Another option is to use the codepage-strings library, which would essentially add support for pretty much every documented codepage out there. The issue is that that crate relies on encoding_rs, which is a pretty heavy crate size-wise, and I am afraid that adding it as a dependency may make it impossible to use this simple-fatfs in embedded systems, the whole reason I created it in the first place.

Considering the fact than unless you are reading a filesystem created in the '90s this change wouldn't impact the end user, I will probably go with the first option. I'd like some feedback if possible though.