Fix default import in TypeScript definitions

[duro]

The current TypeScript definitions throw type errors when the package is imported.

When imported as such:

import DynamoDBFeatureStore from 'launchdarkly-node-server-sdk-dynamodb'

The following type error is emitted from the compiler:

Cannot invoke an expression whose type lacks a call signature. Type 'typeof import("launchdarkly-node-server-sdk-dynamodb")' has no compatible call signatures.

This is because the default import has not been properly tagged, and so typescript thinks that DynamoDBFeatureStore is a named import and wants you to import like so:

import { DynamoDBFeatureStore } from 'launchdarkly-node-server-sdk-dynamodb'

But then at run time, this throws an error because that named import does not actually exist.

This PR correctly tags the function as a default export.

Fixes #10