SPM (Swift Package Manager) support

[nylki]

This PR adds support for installation via SPM (Swift Package Manager) by adding a Package.swift and a required include directory with a symlink to the header file.

I set the minimum swift tools version to 5.4 and minimum iOS target to version 11.0. But feel free change either of those (though iOS 11 is the minimum possible AFAIK).

[jpsim]

Thanks for your contribution! I haven't verified that the approach taken by this library still works in recent iOS versions however.

[nylki]

@jpsim thank you for looking into the PR so quickly! :slightly_smiling_face: For what it's worth, I started using it in iOS 16 in a SwiftUI context and atleast for my use-case, it generally works quite well!

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Just in case you wanna test it yourself on newer iOS version, here are some things I noticed that may not work as intended anymore, but I was able to work around:

• When the system volume is 0 before calling start(), the isAdjustingInitialVolume guard is not working as robust as it might have been in the past: there is still one single unwanted call to the up/down block in that case, but works flawless afterwards.

• not sure if the exactJumpsOnly works properly, did not fire the callbacks reliably when I set it (I ended up not setting it)

• setting the up/down blocks after initializing an instance of the handler did not work for me (they weren't called), but I did not look into it further.

cheers!