Annotations with mixins?

Nobody has claimed this yet.

Hi I'm trying to make annotations that must require the user to have certain fields in a table as a requirement.

For instance, I want a table that needs to have x,y fields, or certain functions.

Here is some pseudo-code that might explain what I mean:
```
---@class Spinnable
---@field spin fun()

---@class Positionable
---@field x number
---@field y number
---@field setPos fun(x: number, y: number)

---@generic T : Spinnable, Positionable
---@param p1 `T`
---@return T
local function doFunction(p1) return p1 end

local entity = { x = 5 } -- A table that is missing the other fields
local check = doFunction(entity) -- I want a warning here saying it doesn't have, `y`, `setPos()`, or `spin()`
```

Is this currently possible to require certain fields in a table?

Start by reproducing the annotated pseudo-code from the issue and review how the language server handles class annotations, generics, and table-field checks. Done means a table missing y, setPos, or spin produces a diagnostic when passed to doFunction, while a compatible table is accepted.

Written by the indexing model from the issue text.