[Diagnostics request] missing module / module not found

From [discussions#2339](https://github.com/LuaLS/lua-language-server/discussions/2339), carsakiller notified me that LuaLS doesn't have this diagnostic. Here's a simple test where I would like there to be a diagnostic message:

```lua
local co = require 'corutine' -- misspelling, `co` has Unknown type
```