Support mode 2031 dark/light mode detection

[the-mikedavis]

The Contour terminal has a VT extension to allow querying for the terminal's color palette preference and to receive status reports when it changes. Historically we haven't wanted to add dark/light mode detection since it would mean adding platform-specific dependencies but with the VT extension we can support it without any extra deps. This will take some work on the terminal manipulation side to support as well as new configuration:

# config.toml
# theme = "default" # this should still be supported
theme = { dark = "catppuccin_frappe", light = "catppuccin_latte" }