Feature Request: ability to overload the return value type

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### How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

macOS

### What is the issue affecting?

Annotations

### Feature request

In my game's ECS bindings I have getter functions that work like this:

```
---@param id integer
---@return number? inv_mass
---@return number? restitution
function ecs.get_collision_body_values(id) end

local inv_mass, restitution = ecs.get_collision_body_values(entity_id)
```

This calls into a native ECS implementation that either pushes the 2 return values and returns with `return 2;` (2 return values) or if the return values doesn't exist, push nothing and `return 0;` (zero return values).

This can be used in my game code like this:

```
local function debug_body(id)
local inv_mass, restitution = ecs.get_collision_body_values(id)
if inv_mass == nil then -- collision_body component not set, do nothing
return
end
---@cast restitution number
-- ^^ have to cast restitution number? -> number to avoid warning
print(inv_mass, restitution)
end
```

Would be nice if there was a way to annotate an "all or nothing" multiple return value type. E.g., a function will either return multiple return values (e.g., not `number?` but `number`) or that it will return nothing. Or I guess more generally, the ability to somehow specify a union of different multivalue return types.

FWIW, it'd be possible to return "a bunch of values" or "nothing" by using a `table?` return type. However, I'm trying very hard not to allocate tables in my hot game's hot paths, thus I favor concrete types and multiple return.

Inizia tracciando il modo in cui il server del linguaggio rappresenta i valori di ritorno multipli annotati e li verifica nei siti di chiamata. Usa l’esempio ECS nell’issue come comportamento da analizzare, inclusi i casi con zero e due valori. Il lavoro è completato quando un’annotazione supportata può esprimere valori di ritorno multivalore all-or-nothing o uniti, senza richiedere il cast mostrato, e la diagnostica è coperta per entrambi i casi.

Scritto dal modello di indicizzazione a partire dal testo della issue.