`fedify node` breaks favicon on terminal emulators without truecolor

[dahlia]

Currently the fedify node command breaks favicon on terminal emulators without 24-bit truecolor.[^1] Here's an example in macOS's built-in Terminal app, which doesn't support truecolor:

For your information, it should be shown like below, which is taken in GNOME Terminal:

We have two options to address it:

1. Properly fix the favicon.

   • Detect if the running terminal emulator supports truecolor.
   • If the terminal emulator does not support it, display favicon in 256 color.

2. Make it hide favicon at all.

   • Detect if the running terminal emulator supports truecolor.
   • If the terminal emulator does not support it, do not display favicon at all.

[^1]: Note that quite many terminal emulators do not support truecolor. See also Terminal Colors.