This adds a flag to define the default behavior for unhandled
rejections. Three modes exist: `none`, `warn` and `strict`. The first
is going to silence all unhandled rejection warnings. The second
behaves identical to the current default with the excetion that no
deprecation warning will be printed and the last is going to throw
an error for each unhandled rejection, just as regular exceptions do.
It is possible to intercept those with the `uncaughtException` hook
as with all other exceptions as well.

This PR has no influence on the existing `unhandledRejection` hook.
If that is used, it will continue to function as before.

PR-URL: nodejs#26599
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

This adds a flag to define the default behavior for unhandled
rejections. Three modes exist: `none`, `warn` and `strict`. The first
is going to silence all unhandled rejection warnings. The second
behaves identical to the current default with the excetion that no
deprecation warning will be printed and the last is going to throw
an error for each unhandled rejection, just as regular exceptions do.
It is possible to intercept those with the `uncaughtException` hook
as with all other exceptions as well.

This PR has no influence on the existing `unhandledRejection` hook.
If that is used, it will continue to function as before.

PR-URL: #26599
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

Notable changes:

* crypto:
  * add support for chacha20-poly1305 for AEAD (chux0519)
    #24081
  * increase maxmem range from 32 to 53 bits (Tobias Nießen)
    #28799
* deps:
  * update npm to 6.11.3 (claudiahdz)
    #29430
  * upgrade openssl sources to 1.1.1d (Sam Roberts)
    #29921
* dns:
  * remove dns.promises experimental warning (cjihrig)
    #26592
* fs:
  * remove experimental warning for fs.promises (Anna Henningsen)
    #26581
* http:
  * makes response.writeHead return the response (Mark S. Everitt)
    #25974
* http2:
  * makes response.writeHead return the response (Mark S. Everitt)
    #25974
* n-api:
  * make func argument of napi\_create\_threadsafe\_function optional
    (legendecas)
    #27791
  * mark version 5 N-APIs as stable (Gabriel Schulhof)
    #29401
  * implement date object (Jarrod Connolly)
    #25917
* process:
  * add --unhandled-rejections flag (Ruben Bridgewater)
    #26599
* stream:
  * implement Readable.from async iterator utility (Guy Bedford)
    #27660
  * make Symbol.asyncIterator support stable (Matteo Collina)
    #26989

PR-URL: #29875

Notable changes:

* crypto:
  * add support for chacha20-poly1305 for AEAD (chux0519)
    #24081
  * increase maxmem range from 32 to 53 bits (Tobias Nießen)
    #28799
* deps:
  * update npm to 6.11.3 (claudiahdz)
    #29430
  * upgrade openssl sources to 1.1.1d (Sam Roberts)
    #29921
* dns:
  * remove dns.promises experimental warning (cjihrig)
    #26592
* fs:
  * remove experimental warning for fs.promises (Anna Henningsen)
    #26581
* http:
  * makes response.writeHead return the response (Mark S. Everitt)
    #25974
* http2:
  * makes response.writeHead return the response (Mark S. Everitt)
    #25974
* n-api:
  * make func argument of napi\_create\_threadsafe\_function optional
    (legendecas)
    #27791
  * mark version 5 N-APIs as stable (Gabriel Schulhof)
    #29401
  * implement date object (Jarrod Connolly)
    #25917
* process:
  * add --unhandled-rejections flag (Ruben Bridgewater)
    #26599
* stream:
  * implement Readable.from async iterator utility (Guy Bedford)
    #27660
  * make Symbol.asyncIterator support stable (Matteo Collina)
    #26989

PR-URL: #29875

--unhandled-rejections has three explicit modes (strict, warn, none)
plus one implicit "default" mode, which logs an additional deprecation
warning (DEP0018).

Prior to this commit, the default mode was subtly different from warn
mode. If the unhandledRejections hook is set, default mode suppresses
all warnings. In warn mode, unhandledRejections would always fire a
warning, regardless of whether the hook was set.

In addition, prior to this commit, strict mode would always throw an
exception, regardless of whether the hook was set.

In this commit, all modes honor the unhandledRejections hook. If the
user has set the hook, then the user has taken full responsibility over
the behavior of unhandled rejections. In that case, no additional
warnings or thrown exceptions will be fired, even in warn mode or
strict mode.

This commit is a stepping stone towards resolving DEP0018. After this
commit, any code that includes an unhandledRejection hook will behave
unchanged when we change the default mode.

Refs: nodejs#26599