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• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
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Description of change

Currently, if I try to initialize getterMethods or setterMethods while initializing a model in a TypeScript project, TS does not infer the correct type for this inside of those methods (and does not allow this to be easily casted to the correct type):

import { Association, HasOne, Model, Sequelize, DataTypes } from 'sequelize';

class MyModel extends Model {
  public num!: number;
}

MyModel.init({}, {
  sequelize,
  tableName: 'my_model',
  getterMethods: {
    multiply: function() {
      return this.num * 2; // <-- ERROR
    }
  }
});