Make isBoom type definition laxer

[nulltoken]

Especially useful in hapijs context, where request.response can be a
Boom object. This change allows the TypeScript code to leverage isBoom()
to check if request.response is a Boom object or not, without any noisy typecast.

[nulltoken]

@cjihrig, @devinivy, @Nargonath, @lloydbenson, @nlf, @geek 👋 kind bump?

[Nargonath]

I'm not a regular TS user but LGTM, let's see what other have to say about it.

[kanongil]

This looks like a very sensible (and backwards compatible) change.

FYI, for TS, you could just use a obj instanceof Boom check instead.

[nlf]

agreed, this looks totally reasonable and how i would expect things to function 👍