Add support for missing 16-bit codepages

I haven't inventoried it, but I suspect some important 16-bit codepages are missing. It would be great to add them either here or to `oem-cp` so that these formats can be dealt with.

Start by inventorying the 16-bit codepages currently supported by codepage-strings and comparing them with the formats that need coverage. Review whether the additions belong here or in `oem-cp`; done means the important missing codepages are supported in the appropriate project and the affected formats can be handled.

Written by the indexing model from the issue text.