Type narrowing for unioned classes

還沒有人認領這個 Issue。

I have a large number of classes that look something like this:
```lua
---@class Foo
---@field type "foo"
---@field name string
---@field property string
-- more unique fields

---@class Bar
---@field type "bar"
---@field name string
---@field stuff string
-- more unique fields

-- more classes
```

I have a function that takes an array of any of these classes as a parameter, so naturally I've unioned all of the classes.
```lua
---@alias Prototypes
---|Foo
---|Bar
-- all of the other classes

---@param prototypes Prototypes[]
local function extend(prototypes)
-- stuff and things
end
```

However, when I try to make `prototypes` a table literal, I get autocomplete for the other classes because the union operator behaves like intersection instead of union.


Is there any way to get it to show only the fields of the class indicated by the `type` field? I can't seem to find way to do a proper union of types.
`@type` seems to only work on variables, and even if it did work on table literals, it would be cumbersome to write out the type for every entry in the array.

首先重現此 issue 中帶註解的 Foo/Bar Prototypes[] 範例，並檢查 language server 的聯合型別縮小與表格字面值補全行為。完成的標準是：依據 type 欄位縮小後的可辨識聯合型別，在補全中只呈現所選類別的欄位，同時保留對其他聯合成員的有效補全。

由索引模型根據 Issue 內容生成。