How do I document a generic class (JS Promises)

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.

Start with the @generic and @class annotations in the Promise example and trace how the language server currently infers callback types through new(), next(), and catch(). Done means determining whether the requested generic propagation is supported or requires a defined type-system change, with the string result inferred in the final callback.

Written by the indexing model from the issue text.