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

Nobody has claimed this yet.

# 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

No files or tests are named. Start by tracing how the language server parses existing @class annotations and inheritance, then determine where ordered @mixin application and field overwriting would fit. Done means the syntax is parsed with the stated precedence rules and covered by tests for multiple mixins and collisions.

Written by the indexing model from the issue text.