Add Mesen emulator

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.

Start with the Mesen and Mesen2 links in the issue, then review how this repository compares existing Game Boy emulators on its test ROMs. Verify Mesen's accuracy and debugging-relevant behavior across the supported platforms, and document the results in the project's comparison output.

Written by the indexing model from the issue text.