breaking: deprecate zenflesh and allow zenbones bg=dark

This is an interim change to prepare removing zenflesh. Please make use
of zenbones and set bg=dark to get the zenflesh colors.

lua/zenbones/init.lua

@@ -1,4 +1,5 @@
 local generator = require "zenbones.specs"
-local p = require "zenbones.palette"
+local bg = vim.opt.background:get()
+local p = require("zenbones.palette")[bg]
 
-return generator.generate(p, "light", generator.get_global_config("zenbones", "light"))
+return generator.generate(p, bg, generator.get_global_config("zenbones", bg))

lua/zenbones/palette.lua

@@ -1,3 +1,8 @@
 local util = require "zenbones.util"
 
-return util.palette_extend({}, "light")
+local M = {}
+
+M.light = util.palette_extend({}, "light")
+M.dark = util.palette_extend {}
+
+return M