Bump version to 8.10 to match current runtime, allow returning result promises in handlers.

There is a tiny paragraph in the documentation describing this new feature, but it is much better described in the blog post announcing it

There is a type-breaking change that should be rare for most users, where previously valid async handlers returning void would be accepted (and their results ignored), I chose to keep this rather than, e.g. also allowing Promise<void> as experimentation shows if the async function returns before the callback is invoked the lambda runtime will return a null value, and it is all too easy to do so. Because of this I chose to (finally!) set the type package version to the runtime version - but I would be happy to back this out if others think it's a minor enough break that needing a package.json update is unnecessary.

Not sure if this should change to referencing/depending on node typings specifically at v8 as well - I think that would cause most users lots of type conflicts right now, but longer term it could catch people using node APIs their runtime doesn't have? If that's happening it should probably happen now so the breaking change can ride the major version bump.

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