Add catppuccin colorscheme, latte in light mode and macchiato in dark

Installed via vim.pack, matching the other plugins here. The pack is
named explicitly ('catppuccin') because the repo is called 'nvim', which
would otherwise be the plugin's name in the lock file and on disk.

flavour = 'auto' with background = { light = 'latte', dark = 'macchiato' }
so the flavour follows 'background', which the TUI sets from the terminal
on startup. It is required first from config/plugins/init.lua so the other
plugins load with the colorscheme already applied.

Verified headlessly in a single nvim: `set background=light` gives
catppuccin-latte and `set background=dark` gives catppuccin-macchiato, so
a running instance can be switched by hand with `:set background=` as the
top-level README already suggests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

neovim/nvim-pack-lock.json

@@ -1,5 +1,9 @@
 {
   "plugins": {
+    "catppuccin": {
+      "rev": "edefef779ab08ce1a4a404713e3012b0d202bd35",
+      "src": "https://github.com/catppuccin/nvim"
+    },
     "diffview.nvim": {
       "rev": "4516612fe98ff56ae0415a259ff6361a89419b0a",
       "src": "https://github.com/sindrets/diffview.nvim"