Feature Request: ability to overload the return value type

還沒有人認領這個 Issue。

### 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.

首先追蹤 language server 如何表示帶註解的多個回傳值，以及如何在呼叫點檢查這些回傳值。使用 issue 中的 ECS 範例作為要調查的行為，包括零值和雙值兩種情況。當受支援的 annotation 能夠表達 all-or-nothing 或合併的多值回傳，而不需要使用所示的 cast，並且兩種情況都有 diagnostics 覆蓋時，即視為完成。

由索引模型根據 Issue 內容生成。