feat(extra): add (better-)discord theme (#662)

## Description

Add a theme for discord. To use this theme, you need a client
implementing support for [better discord
themes](https://betterdiscord.app/)

The theme is a combination of multiple different sources, aiming to
provide an official implementation. Since there seems to be no theme of
this kind currently still being maintained.

I would be happy to in the future address any issues relating to this
theme.

NOTE: I have added a `bg_dark1` color since you need at least 3
bg-colors for 3 different sections of the discord UI from left to right
(see picture). I'm not aware of a specific method to adding colors, so I
just picked some colors that looked right.

## Related Issue(s)

None

## Screenshots

<!-- Add screenshots of the changes if applicable. -->

![tokyonight_moon](https://github.com/user-attachments/assets/5fd1e591-c42b-42ea-b02f-5a1f23ce6f9f)
Tokyo Night moon discord theme.

lua/tokyonight/colors/day.lua

@@ -12,5 +12,6 @@ return function(opts)
 
   Util.invert(colors)
   colors.bg_dark = Util.blend(colors.bg, 0.9, colors.fg)
+  colors.bg_dark1 = Util.blend(colors.bg_dark, 0.9, colors.fg)
   return colors
 end

lua/tokyonight/colors/moon.lua

@@ -2,6 +2,7 @@
 return {
   bg = "#222436",
   bg_dark = "#1e2030",
+  bg_dark1 = "#191B29",
   bg_highlight = "#2f334d",
   blue = "#82aaff",
   blue0 = "#3e68d7",

lua/tokyonight/colors/night.lua

@@ -4,4 +4,5 @@ local ret = vim.deepcopy(require("tokyonight.colors.storm"))
 return vim.tbl_deep_extend("force", ret, {
   bg = "#1a1b26",
   bg_dark = "#16161e",
+  bg_dark1 = "#0C0E14",
 })

lua/tokyonight/colors/storm.lua

@@ -2,6 +2,7 @@
 local ret = {
   bg = "#24283b",
   bg_dark = "#1f2335",
+  bg_dark1 = "#1b1e2d",
   bg_highlight = "#292e42",
   blue = "#7aa2f7",
   blue0 = "#3d59a1",