Add Package.swift

[kean]

• Add Package.swift
• Update Bundle usages when SWIFT_PACKAGE is defined
• Use Objective-C names for classes in storyboards – these don't get mangled

Instead of this (crashes):

customClass="MediaEditorThumbCell" customModule="MediaEditor" customModuleProvider="target"

It's now simply this:

customClass="WPMediaEditorThumbCell"

To test:

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PR submission checklist:

• [ ] I have considered adding unit tests where possible.
• [ ] I have considered adding accessibility improvements for my changes.
• [ ] I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.