add luv library annotations

[flrgh]

This adds annotations for the luv library.

Most everything here was taken from the luv API docs and then annotated with lua-language-server types.

Some of the docs are not clear, so I'm certain there are little mistakes here and there, given that it's such a big API surface.

Most of the docs refer to uv rather than luv, so all types are namespaced under uv.*, but I'm not opposed to changing it if need be.

---

For neovim users, once including this library you can annotate vim.loop to use it :)

---@type uv
local uv = vim.loop