Allow opting out of appearance transition callbacks on presenting view controller

[nolanw]

Summary

Allow PanModalPresentables to opt out of having appearance transition callbacks (viewWillAppear etc.) called on the presenting view controller. (#47)

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've written tests to cover the new code and functionality included in this PR.

Notes

testPresentableYValues fails for me locally, but it wasn't relevant to this PR :)

In #47, there's a request for a reason to avoid these calls. In our case, we're showing the modal with a blurred background, and we wanted the presenting view controller's content to continue updating (we stop updating in viewDidDisappear). I went with a property on PanModalPresentable to avoid breaking existing library users, who may rely on the current behavior.

Thank you for sharing PanModal!

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[nolanw]

(I would prefer not to sign a CLA, as that would allow y'all to relicense these changes under a proprietary license. In lieu, I would be happy to submit a Developer Certificate of Origin or similar.)