Set vim.opt.rtp type to vim.Option

## 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.

lua/lazy/core/loader.lua

@@ -476,6 +476,7 @@ function M.add_to_rtp(plugin)
     table.insert(rtp, idx_after or (#rtp + 1), after)
   end
 
+  ---@type vim.Option
   vim.opt.rtp = rtp
 end