Add allowDuplicates to RavenOptions typescript definition

[Amos47]

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• If you've added code that should be tested, please add tests.
• If you've modified the API (e.g. added a new config or public method), update the docs and TypeScript declaration file.
• Ensure your code lints and the test suite passes (npm test).

Problem

We are missing the definition allowing for the allowDuplicates option in the configuration for typescript projects.

Solution

Add the property to the typescript definition.

[Amos47]

I just realized this get's fixed by #1134.

I'd really appreciate that getting merged.