Add interfaces for all our public APIs

[manucorporat]

Sometimes we remove accidentally methods that our users use:
#11397

Right now we don't have a good way to ensure this from happening. I think the best way is to move all our documentation + public API to TS interfaces as a contract between ionic and our users, this way we can detect at compiler time if the public API changes.

[jgw96]

This issue was moved to ionic-team/ionic-feature-requests#245

[ionitron-bot]

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