feat: Added spec for react native cli and npx

[alexnaiman]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Added spec for standalone react-native's cli and also for when using it from npx(npx react-native [command])
See #87

What is the current behaviour? (You can also link to an open issue here)
No specs for react-native

Additional info:

Some of the generators created, fail sometimes because some of the commands used inside the script timeout. Unfortunately, I couldn't figure out exactly a way to avoid that

[mschrage]

There is is a 5 second timeout by default. I think we might make this configureable in the spec to give autocomplete maintainers more control. cc @falky97

Otherwise, this looks great! Thanks for contributing :)