How do I document a generic class (JS Promises)

Nadie ha tomado este issue todavía.

Hello everyone! First of all, thank you for the amazing work on this server. It's really great and extremelly fast to use!

I've a question about how can I use the type system to properly type a Promise-like table that I'm creating. Right now I need to type this piece of code:

```lua
local Promise = require ".../promise"
Promise:new(function(resolve, reject)
-- do some async stuff...
if some_async_true_status then
resolve(some_async_result)
else
reject(some_async_error)
end
end):next(function(result) -- < `resolve()` value here
print(result)
end):catch(function(err) -- < `reject()` value here
print(err)
end)
```

This a Promise-like implementation that I'm doing for my neovim plugin which I'm trying to get a similar API as the Javascript Promise.

My question here is, how can I type this to the callee function knows what this is going to be returned from that callback? Currently, I'm trying to play around with `@generic` but, I can't get the server to properly understand what I mean by my types:

```lua
local Promise = {}

---@generic T
---@class Promise
---@field new(executor: fun(resolve: fun(result: T), reject: fun(err: any)))): Promise
---@field next(fn: fun(result: T)): Promise <<<<< here I'm returning `self`
---@field catch(fn: fun(err: any)): Promise <<<<< here I'm returning `self` as well

function Promise:new(executor)
local promise = { ... }
-- ... some operations
return setmetatable(promise, {
__index = self,
})
end

function Promise:next(fn)
-- some operations...
return self
end

function Promise:catch(fn)
-- some operations...
return self
end

----------------------
-- Here is how I want to use it:

---@return Promise
function my_function_that_return_promise()
return Promise:new(function(resolve, reject)
-- .....
resolve("hello")
end)
end

my_function_that_return_promise():next(function(result)
print(result) -- << here I want the server to show `result` is a `string`
end)
```

For reference, this is how we can do this exactly thing using typescript:

```typescript
const myPromise = new Promise((resolve, reject) => {
// ...
resolve("hello")
})

myPromise
.then((result) => console.log(result)) // < `result` will have `string` type
.catch((err) => console.log(err))
```

Thank you for the awesome work! Sorry if I didn't make myself clear, let me know if I need to provide any further information.

Comienza con las anotaciones @generic y @class del ejemplo de Promise y sigue cómo el language server infiere actualmente los tipos de callback mediante new(), next() y catch(). La tarea estará terminada cuando se determine si la propagación genérica solicitada es compatible o requiere un cambio definido en el sistema de tipos, con el resultado de tipo string inferido en el callback final.

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