Support tuple/fixed length array types?

It'd be nice to have tuple types or fixed length arrays available as annotations, declaring that a function expects e.g., an array of 3 numbers.

```lua
---@alias vec3 [number, number, number]
---@param a vec3
function length(a)
  return ...
end
```

or 

```lua
---@alias vec3 number[3]
```

See e.g., https://www.typescriptlang.org/docs/handbook/basic-types.html#tuple
