lib: add reason to AbortSignal

[jasnell]

A new reason property was recently added to the AbortSignal spec.

const ac = new AbortController();
ac.abort(new Error('boom!'));
console.loc(ac.signal.reason);  // Error('boom!');

[benjamingr]

Probably needs more docs and tests (and wpts) but can be done later :)

[jasnell]

@nodejs/build ... hey all, are the failures in the arm cluster a known issue?

[jasnell]

Landed in 7633c86