feat: add transparent_warn option in config

[SuXYIO]

So i use transparent.nvim, and when sending a notification, there's a warning about the background color being transparent, which is helpful, but annoying.

I added an option in config (transparent_warn, default true, which is same as the current behavior), which controls whether this warning will be shown.

I tested on my machine and it seems to work fine.

My test config (via Lazy.nvim):

{
	--"rcarriga/nvim-notify",
	dir = "~/Workspace/nvim-notify",
	event = "VeryLazy",
	init = function()
		vim.notify = require("notify")
	end,
	opts = {
		transparent_warn = false,
	},
}

I didn't update the docs though, i'm not good at writing those.