fix(types): use correct this value when initializing getterMethods

[schmod]

Pull Request check-list

Please make sure to review and check all of these items:

• 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?
• Have you added new tests to prevent regressions?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Did you update the typescript typings accordingly (if applicable)?
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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
    }
  }
});

[codecov]

Codecov Report

Merging #11292 into master will increase coverage by 0.76%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11292      +/-   ##
==========================================
+ Coverage   95.58%   96.34%   +0.76%     
==========================================
  Files          91       94       +3     
  Lines        8855     9057     +202     
==========================================
+ Hits         8464     8726     +262     
+ Misses        391      331      -60

Impacted Files	Coverage Δ
lib/dialects/mariadb/index.js	100% <0%> (ø)
lib/dialects/mariadb/connection-manager.js	100% <0%> (ø)
lib/dialects/mariadb/query.js	87.87% <0%> (ø)
lib/dialects/abstract/query-generator.js	97.61% <0%> (+0.17%)	⬆️
lib/sequelize.js	95.95% <0%> (+0.62%)	⬆️
lib/query-interface.js	92.17% <0%> (+1.46%)	⬆️
lib/dialects/mariadb/data-types.js	100% <0%> (+49.01%)	⬆️
lib/dialects/mariadb/query-generator.js	97.87% <0%> (+87.23%)	⬆️

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[SimonSchick]

Suppose.

[sushantdhiman]

🎉 This PR is included in version 5.13.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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