Add Pegasus Frontend

Pegasus is a frontend launcher for game emulators (such as those currently in bucket)

• Files are distributed via GitHub
• Placing "portable.txt" in the application root will cause it to run in portable mode (docs)
• Portable mode configuration files are located in "/config" in the applcation root (docs)
• Possible manifest would create and persist the portable.txt and config folder

this might also be more fitted for Scoop Extra, not confident on which one it fits in better

Done in #711