OnSystem: Add UsesOnSystem class

[samford]

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This adds a UsesOnSystem class to OnSystem, containing boolean instance variables to indicate which types of on_system methods are used in a formula or cask. This is intended as a replacement for @on_system_blocks_exist, which doesn't allow us to determine what kinds of on_system calls were used. This provides more granularity but we can still use @uses_on_system.present? to determine whether any on_system calls were used (and this doubles as a nil check in Formula, as the self.class instance variable has to use a nilable type).

The UsesOnSystem instance variables cover the current ARCH_OPTIONS and BASE_OS_OPTIONS. At the moment, we mostly need to tell whether there are macOS/Linux or Intel/ARM on_system calls, so I've omitted instance variables for specific macOS version until we have a need for them.

As a practical example, if you wanted to determine whether a cask uses Linux on_system calls, you can call cask.uses_on_system.linux?. The linux boolean will be true if the the cask has an on_linux block, an on_system block (which requires Linux), or uses os linux: .... This is something that would be challenging to determine from outside of OnSystem but it's relatively easy to collect the information in OnSystem methods and make it available like this.

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I've set this as a draft for now, so we can discuss whether this is the right approach (I will add tests, documentation comments, etc. once that's settled). There may be room for improvement here but this works how I intended, at least.

Edit: I resolved the Method uses_on_system does not exist on Cask::Cask error by running brew typecheck --update and committing the RBI change (it suddenly occurred to me while I was doing something completely unrelated 😆).

[MikeMcQuaid]

This is great, a much improved API, nice work @samford!

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.