Add bundled themes and write theme files on startup

[mapitman]

Summary

• write bundled theme files to the config themes directory on startup
• embed and ship bundled theme YAMLs (Catppuccin flavors, Dracula, Nord, Tokyo Night, Gruvbox light/dark)
• update docs for theme selection and available built-ins

Testing

• dotnet format
• dotnet test RipSharp.sln
• dotnet build