Add GSE (Game Boy Speedrun Emulator)

[Rangi42]

GSE "is intended as a successor to the Gambatte-Speedrun emulator", which we already include in the shootout.

[CasualPokePlayer]

The main limitation here is GSE does not have CLI support currently. ROMs are only loadable via Open ROM/Recent ROM list/Drag n' drop. I plan on adding CLI support to GSE later, but it's taken a backseat against other priorities (given it has fairly niche usage).