Allow byte order mark at the start of a file

[squeek502]

Currently, the codepoint U+FEFF anywhere is always a compile error (see https://squeek502.github.io/resinator/divergences.html for details). The intention, though is to allow it at the start of a file.

This is not really a big deal currently, though, because UTF-16 is not supported (where the byte order mark can be useful). See https://github.com/squeek502/resinator/issues/5 for UTF-16 support.