Add Mesen emulator

[Rangi42]

Mesen (https://www.mesen.ca, https://github.com/SourMesen/Mesen2) is known as an accurate emulator, it's cross-platform, and people in gbdev use it for its debugging features. It would be worthwhile for us to verify and document its accuracy.

[pinobatch]

Sour went missing several months ago. It might be useful to consider watching the community fork https://github.com/nesdev-org/Mesen2 if that differs noticeably.