Detect and respond to terminal theme mode (light/dark) updates

[the-mikedavis]

recording.mov

Closes #8899, #10281

Historically we haven't wanted to add this because querying OS theme preference required bringing in a bunch of platform-specific deps. A relatively recent VT extension originally proposed by the Contour terminal emulator though lets us query for the terminal's theme mode and receive updates when that changes. This can all be covered cleanly in crossterm with no extra deps. That extension seems to be gaining some steam with Ghostty and Kitty now supporting it on the terminal emulator side and neovim on the client side.

Theme config accepts an optional, extended form (like in #12098):

# this is still accepted:
# theme = "ayu_dark"
theme = { dark = "ayu_dark", light = "ayu_light" }

On startup we query the current theme mode and subscribe to future changes, swapping when we receive an update.

On the terminal emulator side this needs a bit of setup. The latest Kitty version (v0.38.1) supports this but you also need to follow https://sw.kovidgoyal.net/kitty/kittens/themes/#change-color-themes-automatically-when-the-os-switches-between-light-and-dark

This PR needs changes upstream in crossterm - I have a branch https://github.com/the-mikedavis/crossterm/tree/md/theme-mode that I will turn into a PR.