Unable to use `@enum` or `@alias` type in table index (index signature)

I don't know if this is intended or not, but IIRC this is possible in typescript.

```lua
---@enum idx
local enum = { a = 'a', b = 'b' }

---@param tbl table<idx, string>
local function x(tbl) end
```

When calling `x` with a table (e.g. `x({ | })`), I expect to receive completion for `a` and `b`, but I don't get that.