MEGA65: add cartridge support

[lgblgblgb]

MEGA65-core is planning to have its native cartridge support. So far, the limited support for C64 cartridges would not make too much sense for MEGA65 to support, as for being an emulator, a proper C64 emulator for that purpose is a much better choice. However with having the possibility of native cartridges for MEGA65, the situation is already different.

https://mega65.atlassian.net/wiki/spaces/MEGA65/pages/36962324/MEGA65+Style+Cartridge+Work+in+Progress

https://github.com/MEGA65/mega65-rom-public/issues/62

The Commodore "standard" .CRT format may need to be extended for some MEGA65 support. Current CRT format:

https://vice-emu.sourceforge.io/vice_17.html#SEC429

However, at this point, if CRT is supported (because of its future MEGA65 features), why not support standard C64 cartridges as well.

[lgblgblgb]

Partial (?) support of MEGA65 cartridge autoboot is implemented in ROM version 920385 it seems: https://github.com/MEGA65/mega65-rom/releases/tag/v920385

Testing that ROM in Xemu (currently using branch next commit f16e0aede832d13ebf977e09ce17c2954fa1524d) shows, that it basically works, having the binary cartridge attached, on reset or power-on, the cartridge (mandelbrot explorer, native MEGA65 cartridge example) starts.