#1655 Database.backup() typings

[benallfree]

Here's a first cut at #1655. I have some questions:

1. When, exactly, are each of the callback params called?
2. Is Database.backup('foo.db', ()=>console.log('finished')) a complete example, or do step() and finish need to be called?

The docs say

The db.backup(filename, [callback]) shorthand is sufficient for making a backup of a database opened by node-sqlite3.

But later in the docs it says that step(-1) must be called.

Then it also says in different places that finish() is both required and not required.

Clearing up these ambiguities would help the dx.

[daniellockyer]

Closing in favor of #1726