Add SymbOS FNT format support

[mist64]

• Used by SymbOS operating system (www.symbos.org)
• Supports variable-width proportional fonts (1-8 pixels wide, up to 15 rows)
• Strict saver: always writes start_code 0x20 and 96 glyphs
• Permissive loader: accepts any start code and glyph count
• Tested with 53 font files, binary-identical round-trips https://www.symbos.org/media.htm?5