Improved LambdaHandler event typing

[rhys-jmc]

@types/aws-lambda allows the event to be typed as shown here: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/aws-lambda/index.d.ts#L1028

Wrapping the handler with lambdaHandler() removed the @types/aws-lambda typing. Forcing the event to always be of type object breaks the expected type of event in the handler. This change would enable you to type lambdaHandler() similarly to Handler from @types/aws-lambda. It would reinstate the desired type of event for the handler.

It should also be considered to enable typing of the result and to use the Context type from @types/aws-lambda instead of object.

[waltjones]

@VorkS Thanks your for the PR, and the detailed explanation.