Bug: Add charset character to beginning of file for SEQ files

[chriskewl]

Right now when using your graphics on a BBS it is not setting the ABC abc charset. Most BBSs use SEQ files to accomplish this.

We discuss this in another Github issue for a BBS software: https://github.com/retrocomputacion/retrobbs/issues/8#issuecomment-1396418189

What is stated:

Ah, forgot Petmate doesnt save the character to switch charset when exporting to .seq You'll need use an hex editor to insert either $8e for upper/gfx or $0e for upper/lower charset (at the start of the file).

This should be an easy fix that you can do on a SEQ file at the beginning of the file. Thanks!

[chriskewl]

Looks like you fixed something in .c export but not in .seq export: https://github.com/nurpax/petmate/issues/203

Please fix for .seq as well. Thanks!