SchemaType "Set"

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the feature has not already been requested

### 🚀 Feature Proposal

ECMAScript type "Set" can be useful in schema

### Motivation

there are many subdocument schema which should be unique within an individual document.

### Example
```
const abcSchema= new Schema({
name:{type:String, unique: true},
keywords: Set(String)
})
```

No files, tests, or entry points are named. Start by tracing how Mongoose schema types are defined and persisted to MongoDB, then compare the requested Set(String) declaration with existing schema behavior. Done should mean Set is supported in schemas and its uniqueness within an individual document works as described.

Written by the indexing model from the issue text.