refactor: use nvim_set_hl to speed up startup time

On my machine, prior to this change, the colorscheme adds about 8ms to
the startup time of neovim. Using the newly introduced API
`nvim_set_hl`, it’s a little under 3ms, so that’s more than a two times
speedup!

Special care is taken to handle the `fmt` argument, as that’s the least
friendly to `nvim_set_hl`.

This change strives to be backward compatible. Once neovim version 0.7.0
will be widely spread enough, further performance gain are within reach
by:
* removing the fallback that calls `string.format`
* defining the `group_settings` so that those can be passed to `nvim_set_hl` directly

lua/onedark/highlights.lua

@@ -5,7 +5,23 @@ local util = require("onedark.util")
 local M = {}
 local hl = {langs = {}, plugins = {}}
 
-local function vim_highlights(highlights)
+local function vim_highlights_nvim070(highlights)
+    for group_name, group_settings in pairs(highlights) do
+        local settings = {
+            fg = group_settings.fg or "none",
+            bg = group_settings.bg or "none",
+            sp = group_settings.sp or "none",
+        }
+        if not group_settings.fmt == nil then
+            for setting in vim.split(group_settings, ",") do
+                settings[setting] =  1
+            end
+        end
+        vim.api.nvim_set_hl(0, group_name, settings)
+    end
+end
+
+local function vim_highlights_prior_to_nvim070(highlights)
     for group_name, group_settings in pairs(highlights) do
         vim.api.nvim_command(string.format("highlight %s guifg=%s guibg=%s guisp=%s gui=%s", group_name,
             group_settings.fg or "none",
@@ -15,6 +31,11 @@ local function vim_highlights(highlights)
     end
 end
 
+local vim_highlights = vim_highlights_prior_to_nvim070
+if vim.fn.has('nvim-0.7.0') then
+    vim_highlights = vim_highlights_nvim070
+end
+
 local colors = {
     Fg = {fg = c.fg},
     LightGrey = {fg = c.light_grey},