update guidance about plugin config typing

[mrjones2014]

Nowadays, lua_ls supports (partial) classes, basically you can inherit from another class but make all its fields nullable.

For example:

---@class SomeClass
---@field test boolean

---@class (partial) PartialClass: SomeClass

---@type SomeClass
local thing = {} -- warning about missing `test` field

---@type PartialClass
local thing2 = {} -- no warning because `test` field is nullable/optional

I've used this successfully in a few plugins, it works really nicely.

[mrcjkb]

Thanks for your suggestion 🙏

This approach has some potential caveats:

1. Can you generate useful vimdoc from (partial) classes (using panvimdoc or vimcats, etc.)?
   It isn't used anywhere in Neovim core, so I'm guessing there isn't good support for it yet.
   I will have to look into adding support for this to vimcats when I have some time.
2. You have expose the internal class as part of the public API, which may not be worth the reduced boilerplate in some cases.

On the other hand, it potentially means less effort in keeping the classes synchronised.
For now, I'd recommend not to remove the existing section, but state that (partial) classes are an option (while mentioning the caveats).

[mrjones2014]

Good point! I'll fix up the PR some time this week.

[mrjones2014]

Updated