Type narrowing for unioned classes

还没有人认领这个 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.

首先复现此 issue 中带注解的 Foo/Bar Prototypes[] 示例，并检查 language server 的联合类型收窄和表字面量补全行为。完成的标准是：按 type 字段收窄后的可区分联合类型在补全中只显示所选类的字段，同时保留对其他联合成员的有效补全。

由索引模型根据 Issue 内容生成。