fix(util): simplify set_highlight and fix settings being cleared

This function was doing too much unnecessary things and was resetting
user highlights tables to nil as well as unnecessarily trying to merge
tables multiple times this changes should also make this function much
more performant.

lua/astrotheme/lib/util.lua

@@ -55,25 +55,10 @@ function M.set_palettes(opts)
 end
 
 function M.set_highlights(opts, highlights, theme)
-  local opts_hl_glob = opts.highlights.global
-  if opts_hl_glob.modify_hl_groups then
-    opts_hl_glob.modify_hl_groups(highlights, C)
-    opts_hl_glob.modify_hl_groups = nil
-  end
-
-  highlights = vim.tbl_deep_extend("force", highlights, opts_hl_glob)
+  pcall(opts.highlights.global.modify_hl_groups, highlights, C)
+  pcall(opts.highlights[theme].modify_hl_groups, highlights, C)
 
-  local opts_hl = opts.highlights[theme]
-  if opts_hl.modify_hl_groups then
-    opts_hl.modify_hl_groups(highlights, C)
-    opts_hl.modify_hl_groups = nil
-  end
-
-  highlights = vim.tbl_deep_extend("force", highlights, opts_hl)
   for group, spec in pairs(highlights) do
-    for key, value in pairs(spec) do
-      if type(value) == table then spec[key] = value:toHex() end
-    end
     vim.api.nvim_set_hl(0, group, spec)
   end
 end