fix(core): Improve type declaration for model validation functions

[martinslota]

Addresses sequelize#17569 - see also sequelize#17586.

Pull Request Checklist

• Have you added new tests to prevent regressions?

This change is only around the type annotations, so it should not cause any runtime regressions. I'd be happy to add some type-level tests but I'm not really sure how to pull that off in a reasonable way.

• If a documentation update is necessary, have you opened a PR to the documentation repository?
• Did you update the typescript typings accordingly (if applicable)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
• Does the name of your PR follow our conventions?

Description of Changes

Improves the type declaration for model validation functions. Currently the declaration forces validation functions to return a boolean even though that value is never used.

List of Breaking Changes

This should not be a breaking change - except on the type level and only in case the original (faulty) types are already being followed.

[bjornua]

LGTM. This will likely conflict when we try bring our fork up to date