Verification

• This issue's title and/or description do not reference a single formula e.g. brew install wget. If they do, open an issue at https://github.com/Homebrew/homebrew-core/issues/new/choose instead.

Provide a detailed description of the proposed feature

Currently when performing brew upgrade, the quit stanza is skipped.

This FR is to add the ability for cask maintainers to opt-in to forcibly quit before upgrading their apps.

What is the motivation for the feature?

Based on feedback from @bevanjkay on Homebrew/homebrew-cask#173061

In the instance of Gitify, users experience errors/exceptions when the application is upgraded in-place without exiting it first

How will the feature be relevant to at least 90% of Homebrew users?

to be discussed

What alternatives to the feature have been considered?

Adding custom preflight and postflight scripts to individual casks