Support the terminal color palette update notifications protocol (terminal dark/light mode detection)

[yorickvP]

This PR adds support for the color palette update notifications protocol. Changing the terminal color theme will generate a Messages.TerminalColorTheme, and one will be generated at the start.

This can be used to implement auto light/dark theme switching.
In the future, this could be extended to the web and to implement automatic theme switching inside textual (maybe using TEXTUAL_THEME=dark=textual-dark,light=textual-light or something similar).

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[yorickvP]

I'm using this in my own app with

class TApp(App):
    def on_terminal_color_theme(self, message: TerminalColorTheme):
        match message.theme:
            case "light":
                self.theme = "catppuccin-latte" # "textual-light"
            case "dark":
                self.theme = "catppuccin-mocha" # "textual-dark"