[Feature Request] @mixin - @class specific syntax to define applied mixins

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# What
A way to indicate that the members of a `@class` type are applied to a subject `@class` outside of the inheritance chain

# Technical
`@mixin type, ...`
Adds the values of `type, ...` to the subject class' definition. Mixins are applied in the order they are specified; values of the same key are overwrote as each mixin is processed

### Example
This:
```lua
---@class Mixin1
---@field foo number
---@field bar number

---@class Mixin2
---@field bar string
---@field baz string

---@class Subject
---@field key boolean
---@field foo boolean
---@mixin Mixin1, Mixin2
```

Would get interpreted as
```lua
---@class Subject
---@field key boolean (defined by field; wasn't altered by mixins)
---@field foo number (defined by field, overwrote by mixin1, not overwrite by mixin2)
---@field bar string (defined by mixin1, overwrite by mixin2)
---@field baz string (defined by mixin2)
```

# Use case
I currently have sub-classes that get assigned valves from reusable dictionary tables outside their inheritance chain. The only way to document those modifications is to statically define the fields each time a mixin is applied

Non vengono indicati file o test. Inizia tracciando il modo in cui il language server analizza le annotazioni @class esistenti e l’ereditarietà, quindi determina dove si inserirebbero l’applicazione ordinata di @mixin e la sovrascrittura dei campi. Il lavoro è completo quando la sintassi viene analizzata con le regole di precedenza indicate ed è coperta da test per più mixin e collisioni.

Scritto dal modello di indicizzazione a partire dal testo della issue.