Align Callback with @types/aws-lambda

[h-kishi]

Goal

The goal is to align the callback types of @bugsnag/plugin-aws-lambda and @types/aws-lambda.
@types/aws-lambda is often used to develop lambda functions with TypeScript.
The type of lambda callback is defined in @types/aws-lambda, but the type is different from the callback of CallbackHandler in @bugsnag/plugin-aws-lambda.

bugsnag-js/packages/plugin-aws-lambda/types/bugsnag-plugin-aws-lambda.d.ts

Line 7 in 51a4422

type CallbackHandler = (event: any, context: any, callback: (err: Error|null, response: any) => void) => void

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/b9b6b0e9bd5de6ed18be0a3e07816f60fe6f2cfa/types/aws-lambda/handler.d.ts#L171

export type Callback<TResult = any> = (error?: Error | string | null, result?: TResult) => void;

(I guess that this is the cause of #1344)

Testing

I added a test for the pattern of passing a string to a callback.

[johnkiely1]

Hey @h-kishi,

Thanks for this, we will look to get this reviewed as soon as priorities allow.

[imjoehaines]

Thanks @h-kishi!

Can confirm this fixes #1344