Modern C# - .NET 6

[Kermalis]

Pretty busy and this took a lot out of me. So this may linger here for a while, especially since .NET 7 is right around the corner.

Done:

• Updated DLS2, SoundFont2, and EndianBinaryIO to .NET 6 also
• Sanford.Multimedia.Midi is no more. Added a MIDI implementation to the project which we can change to suit our needs
• It builds and things actually work
• Swapped off "commonfiledialog" and "commonfolderdialog" for the winforms ones, but it doesn't change anything

To do:

• Program icon is not embedded in the .exe
• Finish splitting the UI from the core project
• Finish formatting/clean upgrade. This includes renaming engine files
• Test MIDI package
• Test MIDI matching with MP2K
• Players should use ILoadedSong and set it to null if the song is empty/no events. Only MP2K does this right now
• Try to stop using EndianBinaryIO with the GBA engines since it's not necessary
• UI colors are wrong. I think I mistook HSL for HSV, or the HSL code I'm using is wrong (like some github issues I found might suggest)
• Try to use span and nullable in more places. Remove warnings
• Remove MemoryMarshal usage. It won't work if the system endianness doesn't match the game's

Not going to do:

• Split SDAT channel into Channel/ExChannel again. It is a bit of work and unrelated to this port so it will have to wait.
• The program is remaining Windows-only until this is all working. After that, we can try to get a UI using OpenGL or something. But I'm a bit busy for all that, so join the discord if you want to help