mediawiki.api: Add custom Promise types

[Derugon]

Add custom Promise types to mw.Api and mw.Rest to fix #50.

These types provide the missing abort method, and use tuples for callback argument types to help specifying multiple arguments.

Most API/REST response callbacks are changed to be more specific (and fix some wrong types). For migration purpose, potentially breaking changes are listed below:

• Reject/notify callback argument types are specified. Previously the 1st one was always specified as any, now respectively as string/number for most methods.
• assertCurrentUser: The parameter assertUser is only added if the user is not anonymous. Previously the returned parameter type wrongly had it always specified.
• loadMessagesIfMissing: May resolve to undefined if no valid messages were provided.
• saveOptions: May resolve to undefined if no options were provided. Note that if the argument types allow to deduce that at least one option has been provided, the callback argument types are narrowed accordingly.