Let spm deinit take an app back to CocoaPods

Converting an app to SwiftPM with `--deintegrate` now moves the Podfile
to Podfile.bak instead of deleting React Native lines from it (that edit
left invalid Ruby for multi-line calls). `spm deinit` restores the
Podfile, undoes the .gitignore lines the conversion wrote, removes empty
ReactNativeSPM folders, and says to run `pod install`.

The conversion refuses, before changing anything, when the Podfile has
pods that are not React Native or when Podfile.bak already exists.

Author: chrfalch