Dosbox Pure core added

We added a new core, Dosbox Pure. This brings the total number of available cores on Steam to 65 now.

https://store.steampowered.com/app/3005610/RetroArch__DOSBox_Pure/

DOSBox Pure is a hard fork of the multiplatform MS-DOS emulator. It was built by Psyraven in 2020 specifically for RetroArch/Libretro and implements advanced features like save states, an on-screen keyboard, highly customizable controller setup or rewinding. DOSBox Pure aims for simplicity and ease of use.

https://www.youtube.com/watch?v=rHkIz4-SewI