fix: allow returning `Response` objects from function Handlers

[ndhoule]

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Which problem is this pull request solving?

Per the functions documentation, it seems like you should be able to return a standard Response object from function handlers. However, the existing types only allows a user to return a HandlerResponse object.

List other issues or pull requests related to this problem

It looks like https://github.com/netlify/functions/pull/351 also addresses this issue, but that PR is a little stale at this point.

Describe the solution you've chosen

This updates the Handler type to accept both HandlerResponse and Response as a return value.

Checklist

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[Skn0tt]

Hey Nathan, appreciate the PR! The Handler type is for Lambda Compatibility Mode, which doesn't support Response. So the change you're proposing won't work.