fix(types): set this to doc type in SchemaType.prototype.validate()

[vkarpov15]

Fix #12590

Summary

SchemaType should have a reference to the DocType so it knows that type to set this to in when the user does schema.path('mypath').validate(function(v) { ... }). With this PR, the following script compiles fine

import { Schema, SchemaType, InferSchemaType } from 'mongoose';
  
const UserSchema = new Schema({
  _password: String
});

type User = InferSchemaType<typeof UserSchema>;

UserSchema.path('hashed_password').validate(function (v) {
  if (this._password && this._password.length < 8) {
    this.invalidate('password', 'Password must be at least 8 characters.');
  }
});

Examples

[hasezoey]

LGTM, though with a question