Add a system to inform users of breaking changes

[mrjones2014]

An example: https://github.com/nvim-lualine/lualine.nvim/blob/master/lua/lualine/utils/notices.lua

Maybe just show a message via vim.api.nvim_err_writeln when there are breaking changes, and inform the user to run a (new) command like :DashNotices that will show any new breaking changes.

[mrjones2014]

@wookayin will appreciate this 😉