feat: support dim opacity

[mikesmithgh]

printf "%s\033[2;39m%s" "hi" "bye"

SGR (Select Graphic Rendition) parameters (see https://en.wikipedia.org/wiki/ANSI_escape_code)

2 | Faint, decreased intensity, or dim | May be implemented as a light font weight like bold.[39]
2 Faint, decreased intensity, or dim May be implemented as a light font weight like bold.[39]

kitty.conf

# dim_opacity 0.4

#: How much to dim text that has the DIM/FAINT attribute set. One
#: means no dimming and zero means fully dimmed (i.e. invisible).

kitty with default dim_opacity 0.4

kitty-scrollback.nvim

Determine CSI 2 (faint) color based on kitty.conf dim_opacity, calculate via faint() (See
https://github.com/kovidgoyal/kitty/blob/master/kittens/tui/operations.py#L204).

If too hard to automatically convert, then add documentations and workaround steps.

[mikesmithgh]

kovidgoyal/kitty#6803 may be related, double check once you start to work this