Added ability to specify custom filename during model generation.

[timkatee]

Added ability to specify custom filename during model generation. Developers can therefore ensure consistency for their naming conventions.

Pull Request check-list

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

• [x] Does npm run test pass with this change (including linting)?
• [x] Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
• [x] Have you added new tests to prevent regressions?
• [x] Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

Description of change

[WikiRik]

This is a nice addition! Could you add some tests to this? And maybe some additional documentation to https://github.com/sequelize/sequelize/blob/main/docs/manual/other-topics/migrations.md

[timkatee]

Sure @WikiRik let me do...

[WikiRik]

Thanks! In the meantime we have moved the docs file to https://github.com/sequelize/website/blob/main/docs/other-topics/migrations.md btw