[FEAT]: Show diagnostics option

[adonespitogo]

Requirements

• This feature isn't just a vague idea and can have a good practical solution.
• I'm completely sure that this feature fits into this plugin.
• I'm willing to implement this through a PR.

Problem

I'm using winbar to work with multiple windows. I want to able to see if a certain file has errors at a glance.

Solution Suggestion

Since we have a show_modified option, I think it's also good to have show_diagnostics option.

Workaround

Using the custom_section to show a text or symbol when there are diagnostics error:

-- winbar plugin
return {
	"utilyre/barbecue.nvim",
	name = "barbecue",
	version = "*",
	dependencies = {
		"SmiteshP/nvim-navic",
		"nvim-tree/nvim-web-devicons", -- optional dependency
	},
	config = function()
		local function has_diagnostics(buf)
			local severties = { "error", "warning" }
			for _, sev in ipairs(severties) do
				local n = #vim.diagnostic.get(buf, { severity = vim.diagnostic.severity[string.upper(sev)] })
				if n > 0 then
					return true
				end
			end
			return false
		end

		require("barbecue").setup({
			custom_section = function(bfnr, _)
				local custom = ""
				if has_diagnostics(bfnr) then
					custom = " " .. custom
				end
				return custom
			end,
		})
	end,
}