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.

lua/lazy/core/config.lua

@@ -237,6 +237,7 @@ function M.setup(opts)
   M.me = debug.getinfo(1, "S").source:sub(2)
   M.me = Util.norm(vim.fn.fnamemodify(M.me, ":p:h:h:h:h"))
   if M.options.performance.rtp.reset then
+    ---@type vim.Option
     vim.opt.rtp = {
       vim.fn.stdpath("config"),
       vim.fn.stdpath("data") .. "/site",

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
 

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