fix: supports getting of user-defined color palettes

[misumisumi]

Due to the change in a631be2, an error will now occur if a color scheme other than catppuccin is used.
Previously, when using another color scheme, it seemed to fall back to the defined palette.
This change supports retrieval of user-defined color palettes, allowing you to use a wider range of color schemes.

• error message
  

[ayamir]

My fault, I didn't test the case when colorscheme is not catppuccin.
But the current impl doesn't seem to solve the problem:

And the introduction of get_palette_func seems to be duplicated with palette_overwrite?

[ayamir]

This impl is enough for addressing the current bug:

---Initialize the palette
---@return table
local function init_palette()
	-- Reinitialize the palette on event `ColorScheme`
	if not _has_autocmd then
		_has_autocmd = true
		vim.api.nvim_create_autocmd("ColorScheme", {
			group = vim.api.nvim_create_augroup("__builtin_palette", { clear = true }),
			pattern = "*",
			callback = function()
				palette = nil
				init_palette()
				-- Also refresh hard-coded hl groups
				M.gen_alpha_hl()
				M.gen_lspkind_hl()
				pcall(vim.cmd.AlphaRedraw)
			end,
		})
	end

	local fallback_palette = {
		rosewater = "#DC8A78",
		flamingo = "#DD7878",
		mauve = "#CBA6F7",
		pink = "#F5C2E7",
		red = "#E95678",
		maroon = "#B33076",
		peach = "#FF8700",
		yellow = "#F7BB3B",
		green = "#AFD700",
		sapphire = "#36D0E0",
		blue = "#61AFEF",
		sky = "#04A5E5",
		teal = "#B5E8E0",
		lavender = "#7287FD",

		text = "#F2F2BF",
		subtext1 = "#BAC2DE",
		subtext0 = "#A6ADC8",
		overlay2 = "#C3BAC6",
		overlay1 = "#988BA2",
		overlay0 = "#6E6B6B",
		surface2 = "#6E6C7E",
		surface1 = "#575268",
		surface0 = "#302D41",

		base = "#1D1536",
		mantle = "#1C1C19",
		crust = "#161320",
	}

	if not palette then
		if vim.g.colors_name == nil or (vim.g.colors_name ~= nil and not vim.g.colors_name:find("catppuccin")) then
			palette = fallback_palette
		else
			palette = require("catppuccin.palettes").get_palette()
		end

		palette = vim.tbl_extend("force", { none = "NONE" }, palette, require("core.settings").palette_overwrite)
	end

	return palette
end

[misumisumi]

Oh, I overlooked palette_overwrite.
I removed duplicate settings and changed code.

[ayamir]

LGTM