Pending migration alert during app startup

Review the Sequelize CLI migration commands and the application's Sequelize initialization flow; no specific files, tests, or entry points are named in the issue. Define how pending migrations are detected, which configuration controls the check, and how startup failure and messaging should behave, then add coverage for the chosen behavior.

Issue Creation Checklist

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Feature Description

Pending migration (if any) alert during app startup.

Describe the feature you'd like to see implemented

I checked sequelize docs but didn't find any way of getting notified if there is pending migration left. This could be useful in many ways when there are multiple devs working on the project.
During app startup, upon initialization of sequelize we should check for current migrations in DB and existing migrations in the migrations folder, and upon any mismatch we throw a non-zero exit and stop the application with a message. All this can be configurable IMO.

Describe why you would like this feature to be added to Sequelize

We can have an external npm package for this, but make sense to have it here. Also, this feature is available in Ruby on Rails ActiveRecord gem.

Is this feature dialect-specific?

• No. This feature is relevant to Sequelize as a whole.
• Yes. This feature only applies to the following dialect(s):

Would you be willing to resolve this issue by submitting a Pull Request?

• Yes, I have the time and I know how to start.
• Yes, I have the time but I will need guidance.
• No, I don't have the time, but my company or I are supporting Sequelize through donations on OpenCollective.
• No, I don't have the time, and I understand that I will need to wait until someone from the community or maintainers is interested in implementing my feature.

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