fix(util): replace vim.fn.readfile with io.open

lua/util.lua

@@ -132,7 +132,12 @@ function Util.format(opts)
 
     local tmp_out
     if tmp then
-        tmp_out = table.concat(vim.fn.readfile(tmp), "\n")
+        local f = io.open(tmp, "r")
+        if not f then
+            return
+        end
+        tmp_out = f:read("*a")
+        f:close()
         os.remove(tmp)
     end