Summary:
Add `AlertIOS.prompt`
It's compatible with the js spec, with the exception that I had to add
a callback param since it's async. Also supports the same button configuration
as `AlertIOS.alert`.
@public
Test Plan:
I've updated the `AlertIOS` example on UIExplorer with every
valid combination of
parameters, so just going through it should be fine.
ef339250b5