style: Set vim.opt.rtp type to vim.Option (#1498)

## Details

There is an issue in the `neodev` repo that mentions this:
[#193](https://github.com/folke/neodev.nvim/issues/193)

I think the problem comes from a combination of 2 things:

1. As mentioned in the [Reddit post](https://www.reddit.com/r/neovim/comments/1cvrilk/diagnosticwarning_after_upgrade_to_neovim_010/):
   `Nvim never had Lua type annotations for vim.opt`.
2. Setting `vim.opt.rtp` anywhere in the config will cause Lua-LS to infer the
   type for `vim.opt.rtp`. While users are unlikely to do this it does happen
   in `lazy.nvim`, in places shown in this PR.

tests/core/util_spec.lua

@@ -5,6 +5,7 @@ local Util = require("lazy.util")
 describe("util", function()
   local rtp = vim.opt.rtp:get()
   before_each(function()
+    ---@type vim.Option
     vim.opt.rtp = rtp
     for k, v in pairs(package.loaded) do
       if k:find("^foobar") then