fix: update isHttpError return type

[princemuel]

Hi there! 👋🏼 Please I'm using this package in one my projects and it's really nice.
I had an issue tho' where I'm doing this...

export function handleError<T extends unknown>(exception: T) {
  //  getting this error: Property 'expose' does not exist on type 'T'
  //  since the return type of the isHttpError function is boolean 
  //  not the more stricter HttpError which has the expose property
  if (isHttpError(exception) && exception.expose) { 

  }

and I'd like to contribute if you're open to it.

I replaced the return type of the isHttpError type guard with the type predicate error is HttpError instead of the looser boolean type.
This allows the error have the types of the HttpError class.

[codecov-commenter]

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[ShogunPanda]

Sorry for the long wait. This looks fine to me!